U.S. patent number 7,929,342 [Application Number 11/996,711] was granted by the patent office on 2011-04-19 for magnetic memory cell, magnetic random access memory, and data read/write method for magnetic random access memory.
This patent grant is currently assigned to NEC Corporation. Invention is credited to Shunsuke Fukami, Nobuyuki Ishiwata, Hideaki Numata, Norikazu Ohshima, Tadahiko Sugibayashi, Tetsuhiro Suzuki.
United States Patent |
7,929,342 |
Numata , et al. |
April 19, 2011 |
Magnetic memory cell, magnetic random access memory, and data
read/write method for magnetic random access memory
Abstract
The present invention provides a new data writing method for an
MRAM which can suppress deterioration of a tunnel barrier layer. A
magnetic memory cell 1 has a magnetic recording layer 10 and a
pinned layer 30 connected to the magnetic recording layer 10
through a non-magnetic layer 20. The magnetic recording layer 10
includes a magnetization switching region 13, a first magnetization
fixed region 11 and a second magnetization fixed region 12. The
magnetization switching region 13 has reversible magnetization and
faces the pinned layer 30. The first magnetization fixed region 11
is connected to a first boundary B1 of the magnetization switching
region 13 and its magnetization direction is fixed to a first
direction. The second magnetization fixed region 12 is connected to
a second boundary B2 of the magnetization switching region 13 and
its magnetization direction is fixed to a second direction. Both of
the first direction and the second direction are toward the
magnetization switching region 13 or away from the magnetization
switching region 13.
Inventors: |
Numata; Hideaki (Tokyo,
JP), Ohshima; Norikazu (Tokyo, JP), Suzuki;
Tetsuhiro (Tokyo, JP), Sugibayashi; Tadahiko
(Tokyo, JP), Ishiwata; Nobuyuki (Tokyo,
JP), Fukami; Shunsuke (Tokyo, JP) |
Assignee: |
NEC Corporation (Tokyo,
JP)
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Family
ID: |
37757484 |
Appl.
No.: |
11/996,711 |
Filed: |
August 4, 2006 |
PCT
Filed: |
August 04, 2006 |
PCT No.: |
PCT/JP2006/315528 |
371(c)(1),(2),(4) Date: |
January 24, 2008 |
PCT
Pub. No.: |
WO2007/020823 |
PCT
Pub. Date: |
February 22, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100142264 A1 |
Jun 10, 2010 |
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Foreign Application Priority Data
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Aug 15, 2005 [JP] |
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2005-235187 |
Mar 28, 2006 [JP] |
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2006-088068 |
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Current U.S.
Class: |
365/171; 257/421;
365/158; 365/189.14 |
Current CPC
Class: |
H01L
27/228 (20130101); G11C 11/1675 (20130101); G11C
19/0808 (20130101); G11C 11/1655 (20130101); H01L
43/08 (20130101); G11C 11/1659 (20130101); G11C
11/161 (20130101) |
Current International
Class: |
G11C
11/14 (20060101) |
Field of
Search: |
;365/171,158,189.14
;257/421 |
References Cited
[Referenced By]
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Other References
K Yagami and Y. Suzuki, Research Trends in Spin Transfer
Magnetization Switching. cited by other .
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Domain Wall Motion in Submicron Magnetic Wires, The American
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Primary Examiner: Dinh; Son
Assistant Examiner: Le; Toan
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
The invention claimed is:
1. A magnetic memory cell comprising: a magnetic recording layer
that is a ferromagnetic layer; and a pinned layer connected to said
magnetic recording layer through a non-magnetic layer, wherein said
magnetic recording layer includes: a magnetization switching region
having reversible magnetization and facing said pinned layer; a
first magnetization fixed region connected to a first boundary of
said magnetization switching region and whose magnetization
direction is fixed to a first direction; and a second magnetization
fixed region connected to a second boundary of said magnetization
switching region and whose magnetization direction is fixed to a
second direction, wherein said first magnetization fixed region and
said second magnetization fixed region are formed parallel to each
other, wherein said first direction and said second direction are
the same direction, wherein magnetization direction of said
magnetization switching region is toward any of said first boundary
and said second boundary, and a domain wall is formed at any of
said first boundary and said second boundary in said magnetic
recording layer.
2. The magnetic memory cell according to claim 1, wherein said
magnetization switching region, said first magnetization fixed
region and said second magnetization fixed region are so formed as
to have a U-shape.
3. The magnetic memory cell according to claim 1, wherein said
magnetization switching region, said first magnetization fixed
region and said second magnetization fixed region are formed in a
same plane.
4. The magnetic memory cell according to claim 1, wherein said
magnetization switching region is formed parallel to a first plane,
while said first magnetization fixed region and said second
magnetization fixed region are formed perpendicular to said first
plane.
5. The magnetic memory cell according to claim 4, wherein said
magnetization switching region is formed on a bottom surface of a
trench section, while said first magnetization fixed region and
said second magnetization fixed region are respectively formed on
opposed side surfaces of said trench section.
6. The magnetic memory cell according to claim 1, further
comprising: a first magnetic body configured to apply a bias
magnetic field of said first direction to said first magnetization
fixed region; and a second magnetic body configured to apply a bias
magnetic field of said second direction to said second
magnetization fixed region.
7. The magnetic memory cell according to claim 6, wherein said
first magnetic body and said second magnetic body are so provided
as to be in contact with said first magnetization fixed region and
said second magnetization fixed region, respectively, wherein
magnetization direction of said first magnetic body is said first
direction and magnetization direction of said second magnetic body
is said second direction.
8. The magnetic memory cell according to claim 6, wherein said
first magnetic body and said second magnetic body are so provided
as to be apart from said first magnetization fixed region and said
second magnetization fixed region, respectively.
9. The magnetic memory cell according to claim 8, wherein said
first magnetic body and said second magnetic body are provided
above or below said first magnetization fixed region and said
second magnetization fixed region, respectively, wherein
magnetization direction of said first magnetic body is opposite to
said first direction and magnetization direction of said second
magnetic body is opposite to said second direction.
10. The magnetic memory cell according to claim 8, wherein said
first magnetic body and said second magnetic body are provided in a
same plane as that of said magnetic recording layer.
11. The magnetic memory cell according to claim 10, wherein said
first magnetic body and said second magnetic body are so provided
as to sandwich said magnetic recording layer from both sides,
wherein magnetization direction in an end section of said first
magnetic body closest to said first magnetization fixed region is
said first direction and magnetization direction in an end section
of said second magnetic body closest to said second magnetization
fixed region is said second direction.
12. The magnetic memory cell according to claim 1, wherein said
first magnetization fixed region and said second magnetization
fixed region have magnetic anisotropy of a same direction, wherein
said magnetization switching region has magnetic anisotropy of a
direction different from that of said first magnetization fixed
region and said second magnetization fixed region.
13. The magnetic memory cell according to claim 12, wherein an
external magnetic field whose direction is the same as said first
direction and said second direction is applied.
14. The magnetic memory cell according to claim 1, wherein a
longitudinal direction of said first magnetization fixed region and
a longitudinal direction of said second magnetization fixed region
are the same, wherein a longitudinal direction of said
magnetization switching region is different from the longitudinal
direction of said first magnetization fixed region and said second
magnetization fixed region.
15. The magnetic memory cell according to claim 1, wherein in a
first write operation, a first write current is flowed from said
first magnetization fixed region through said magnetization
switching region to said second magnetization fixed region, wherein
in a second write operation, a second write current is flowed from
said second magnetization fixed region through said magnetization
switching region to said first magnetization fixed region.
16. The magnetic memory cell according to claim 15, wherein a
domain wall is formed at said first boundary in said magnetic
recording layer due to said first write operation, wherein a domain
wall is formed at said second boundary in said magnetic recording
layer due to said second write operation.
17. The magnetic memory cell according to claim 15, wherein said
first direction is toward said first boundary and said second
direction is toward said second boundary, wherein magnetization of
said magnetization switching region turns toward said first
boundary due to said first write operation, wherein magnetization
of said magnetization switching region turns toward said second
boundary due to said second write operation.
18. The magnetic memory cell according to claim 15, wherein said
first direction is away from said first boundary and said second
direction is away from said second boundary, wherein magnetization
of said magnetization switching region turns toward said second
boundary due to said first write operation, wherein magnetization
of said magnetization switching region turns toward said first
boundary due to said second write operation.
19. The magnetic memory cell according to claim 15, further
comprising an assist wiring so formed as to intersect with said
magnetization switching region, wherein a direction of a magnetic
field applied to said magnetization switching region due to a
current flowing through said assist wiring in said first write
operation is toward one of said first boundary and said second
boundary, wherein a direction of a magnetic field applied to said
magnetization switching region due to a current flowing through
said assist wiring in said second write operation is toward the
other of said first boundary and said second boundary.
20. The magnetic memory cell according to claim 19, wherein said
assist wiring is connected to said first magnetization fixed region
or said second magnetization fixed region, wherein in said first
write operation, said first write current flows through said assist
wiring, wherein in said second write operation, said second write
current flows through said assist wiring.
21. The magnetic memory cell according to claim 19, wherein said
assist wiring includes: a first assist wiring formed below said
magnetization switching region; and a second assist wiring formed
above said magnetization switching region.
22. The magnetic memory cell according to claim 1, wherein in a
first write operation, a first write magnetic field is applied to
said magnetization switching region, wherein in a second write
operation, a second write magnetic field whose direction is
opposite to that of said first write magnetic field is applied to
said magnetization switching region.
23. The magnetic memory cell according to claim 1, wherein in a
read operation, a read current is flowed between said pinned layer
and any of said first magnetization fixed region and said second
magnetization fixed region through said magnetization switching
region and said non-magnetic layer.
24. The magnetic memory cell according to claim 1, wherein said
magnetic recording layer includes a plurality of ferromagnetic
layers which are anti-ferromagnetically coupled, wherein a
ferromagnetic layer among said plurality of ferromagnetic layers
adjacent to said pinned layer through said non-magnetic layer
includes said magnetization switching region, said first
magnetization fixed region and said second magnetization fixed
region.
25. The magnetic memory cell according to claim 1, wherein said
magnetic recording layer further includes: another magnetization
switching region having reversible magnetization; and a third
magnetization fixed region whose magnetization direction is fixed
to a third direction, wherein said another magnetization switching
region is connected to said second magnetization fixed region at a
third boundary and is connected to said third magnetization fixed
region at a fourth boundary, wherein said first magnetization fixed
region, said second magnetization fixed region and said third
magnetization fixed region are formed parallel to each other,
wherein said first direction, said second direction and said third
direction are the same direction.
26. The magnetic memory cell according to claim 25, wherein said
another magnetization switching region is connected to another
pinned layer through another non-magnetic layer.
27. The magnetic memory cell according to claim 1, wherein said
magnetization switching region is made of soft magnetic
material.
28. The magnetic memory cell according to claim 27, wherein said
soft magnetic material is microcrystalline in which grain diameter
is not more than a film thickness, or amorphous.
29. The magnetic memory cell according to claim 1, wherein said
magnetization switching region includes at least one element
selected from the group consisting of Co, Fe and Ni.
30. The magnetic memory cell according to claim 29, wherein
composition of said magnetization switching region is represented
by XX-YY-ZZ, wherein said XX includes at least one element selected
from the group consisting of Co, Fe and Ni, said YY includes at
least one element selected from the group consisting of Al, Si, Mg,
Ta, Nb, Zr, Hf, W, Mo, Ti and V, and said ZZ includes at least one
element selected from the group consisting of N, C, B and O.
31. The magnetic memory cell according to claim 29, wherein
composition of said magnetization switching region is represented
by XX-YY, wherein said XX includes at least one element selected
from the group consisting of Co, Fe and Ni, and said YY includes at
least one element selected from the group consisting of Al, Si, Mg,
Ta, Nb, Zr, Hf, W, Mo, Ti and V.
32. The magnetic memory cell according to claim 29, wherein
composition of said magnetization switching region is represented
by XX ZZ, wherein said XX includes at least one element selected
from the group consisting of Co, Fe and Ni, and said ZZ includes at
least one element selected from the group consisting of N, C, B and
O.
33. A magnetic memory cell comprising: a magnetic recording layer
that is a ferromagnetic layer; and a pinned layer connected to said
magnetic recording layer through a non-magnetic layer, wherein said
magnetic recording layer includes: a magnetization switching region
having reversible magnetization and facing said pinned layer; a
first magnetization fixed region connected to a first boundary of
said magnetization switching region and whose magnetization
direction is fixed to a first direction; and a second magnetization
fixed region connected to a second boundary of said magnetization
switching region and whose magnetization direction is fixed to a
second direction, wherein said magnetization switching region, said
first magnetization fixed region and said second magnetization
fixed region are linearly formed in a same plane, wherein said
first direction is opposite to said second direction, wherein
magnetization direction of said magnetization switching region is
toward any of said first boundary and said second boundary, and a
domain wall is formed at any of said first boundary and said second
boundary in said magnetic recording layer, wherein a
cross-sectional area of said magnetization switching region
increases with distance from said first boundary and said second
boundary.
34. The magnetic memory cell according to claim 33, wherein in a
plane parallel to said first boundary and said second boundary,
cross-sectional areas of said first magnetization fixed region and
said second magnetization fixed region are smaller than a
cross-sectional area of said magnetization switching region.
35. The magnetic memory cell according to claim 33, wherein said
magnetic recording layer further includes: another magnetization
switching region having reversible magnetization; and a third
magnetization fixed region whose magnetization direction is fixed
to a third direction, wherein said another magnetization switching
region is connected to said second magnetization fixed region at a
third boundary and is connected to said third magnetization fixed
region at a fourth boundary, wherein said magnetization switching
region, said another magnetization switching region, said first
magnetization fixed region, said second magnetization fixed region
and said third magnetization fixed region are linearly formed,
wherein said first direction is opposite to said second direction,
and said first direction and said third direction are the same.
36. A magnetic random access memory comprising: a magnetic memory
cell; a word line connected to said magnetic memory cell; and a bit
line connected to said magnetic memory cell, wherein said magnetic
memory cell comprises: a magnetic recording layer that is a
ferromagnetic layer; and a pinned layer connected to said magnetic
recording layer through a non-magnetic layer, wherein said magnetic
recording layer includes: a magnetization switching region having
reversible magnetization and facing said pinned layer; a first
magnetization fixed region connected to a first boundary of said
magnetization switching region and whose magnetization direction is
fixed to a first direction; and a second magnetization fixed region
connected to a second boundary of said magnetization switching
region and whose magnetization direction is fixed to a second
direction, wherein said first magnetization fixed region and said
second magnetization fixed region are formed parallel to each
other, wherein said first direction and said second direction are
the same direction, wherein magnetization direction of said
magnetization switching region is toward any of said first boundary
and said second boundary, and a domain wall is formed at any of
said first boundary and said second boundary in said magnetic
recording layer.
37. A data read and write method for a magnetic random access
memory having a magnetic memory cell, said magnetic memory cell
comprising: a magnetic recording layer that is a ferromagnetic
layer; and a pinned layer connected to said magnetic recording
layer through a non-magnetic layer, wherein said magnetic recording
layer includes: a magnetization switching region facing said pinned
layer, in which a domain wall moves; a first magnetization fixed
region connected to a first boundary of said magnetization
switching region and whose magnetization direction is fixed to a
first direction; and a second magnetization fixed region connected
to a second boundary of said magnetization switching region and
whose magnetization direction is fixed to a second direction,
wherein said first magnetization fixed region and said second
magnetization fixed region are formed parallel to each other,
wherein said first direction and said second direction are the same
direction, wherein magnetization direction of said magnetization
switching region is toward any of said first boundary and said
second boundary, and a domain wall is formed at any of said first
boundary and said second boundary in said magnetic recording layer,
said data read and write method comprising: (A) moving said domain
wall to said first boundary by supplying a first write current from
said first magnetization fixed region to said second magnetization
fixed region, when writing a first data; and (B) moving said domain
wall to said second boundary by supplying a second write current
from said second magnetization fixed region to said first
magnetization fixed region, when writing a second data.
38. The data read and write method according to claim 37, further
comprising: (C) flowing a read current between said pinned layer
and any of said first magnetization fixed region and said second
magnetization fixed region through said magnetization switching
region and said non-magnetic layer, when reading said first data or
said second data stored in said magnetic memory cell.
39. A magnetic memory cell comprising: a magnetic recording layer
that is a ferromagnetic layer; a pinned layer connected to said
magnetic recording layer through a non-magnetic layer; and an
assist wiring, wherein said magnetic recording layer includes: a
magnetization switching region having reversible magnetization and
facing said pinned layer; a first magnetization fixed region
connected to a first boundary of said magnetization switching
region and whose magnetization direction is fixed to a first
direction; and a second magnetization fixed region connected to a
second boundary of said magnetization switching region and whose
magnetization direction is fixed to a second direction, wherein
said first magnetization fixed region and said second magnetization
fixed region are formed parallel to each other, wherein said assist
wiring is so formed as to intersect with said magnetization
switching region, wherein in a first write operation, a direction
of a magnetic field applied to said magnetization switching region
due to a current flowing through said assist wiring is toward one
of said first boundary and said second boundary, wherein in a
second write operation, a direction of a magnetic field applied to
said magnetization switching region due to a current flowing
through said assist wiring is toward the other of said first
boundary and said second boundary.
40. The magnetic memory cell according to claim 39, wherein said
assist wiring is connected to said first magnetization fixed region
or said second magnetization fixed region, wherein in said first
write operation, a first write current flows through said assist
wiring, wherein in said second write operation, a second write
current whose direction is opposite to that of said first write
current flows through said assist wiring.
41. The magnetic memory cell according to claim 39, wherein said
assist wiring includes: a first assist wiring formed below said
magnetization switching region; and a second assist wiring formed
above said magnetization switching region.
42. The magnetic memory cell according to claim 39, wherein in said
first write operation, a first write magnetic field is applied to
said magnetization switching region, wherein in said second write
operation, a second write magnetic field whose direction is
opposite to that of said first write magnetic field is applied to
said magnetization switching region.
43. The magnetic memory cell according to claim 39, wherein said
magnetic recording layer includes a plurality of ferromagnetic
layers which are anti-ferromagnetically coupled, wherein a
ferromagnetic layer among said plurality of ferromagnetic layers
adjacent to said pinned layer through said non-magnetic layer
includes said magnetization switching region, said first
magnetization fixed region and said second magnetization fixed
region.
Description
TECHNICAL FIELD
The present invention relates to a magnetic random access memory in
which magnetic memory cells are integrated, and a data read/write
method for the magnetic random access memory.
BACKGROUND ART
An MRAM (Magnetic Random Access Memory) is a promising nonvolatile
memory from a viewpoint of high integration and high-speed
operation. In the MRAM, a magnetoresistance element that exhibits a
"magnetoresistance effect" such as TMR (Tunnel MagnetoResistance)
effect is utilized. In the magnetoresistance element, for example,
an MTJ (Magnetic Tunnel Junction) in which a tunnel barrier layer
is sandwiched by two ferromagnetic layers is formed. The two
ferromagnetic layers include a pinned layer whose magnetization
direction is fixed and a free layer whose magnetization direction
is reversible.
It is known that a resistance value (R+.DELTA.R) of the MTJ when
the magnetization directions of the pinned layer and the free layer
are "anti-parallel" to each other becomes larger than a resistance
value (R) when the magnetization directions are "parallel" to each
other due to the magnetoresistance effect. The MRAM uses the
magnetoresistance element having the MTJ as a memory cell and
nonvolatilely stores data by utilizing the change in the resistance
value. Data writing to the memory cell is performed by switching
the magnetization direction of the free layer.
Conventionally known methods of data writing to the MRAM include an
"asteroid method" disclosed for example in U.S. Pat. No. 5,640,343
and a "toggle method" disclosed for example in U.S. Pat. No.
6,545,906 and National Publication of the Translated Version of PCT
Application JP-2005-505889. According to these write methods, a
magnetic switching field necessary for switching the magnetization
of the free layer increases in substantially inverse proportion to
the size of the memory cell. That is to say, a write current tends
to increase with the miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write
current with the miniaturization, there is proposed a "spin
transfer method" as disclosed in Japanese Laid-Open Patent
Application JP-2005-093488 and "Yagami and Suzuki, Research Trends
in Spin Transfer Magnetization Switching, Journal of The Magnetics
Society of Japan, Vol. 28, No. 9, 2004. According to the spin
transfer method, a spin-polarized current is injected to a
ferromagnetic conductor, and direct interaction between spin of
conduction electrons of the current and magnetic moment of the
conductor causes the magnetization to be switched (hereinafter
referred to as "Spin Transfer Magnetization Switching"). The spin
transfer magnetization switching will be outlined below with
reference to FIG. 1.
In FIG. 1, a magnetoresistance element is provided with a free
layer 101, a pinned layer 103, and a tunnel barrier layer 102 that
is a non-magnetic layer sandwiched between the free layer 101 and
the pinned layer 103. Here, the pinned layer 103, whose
magnetization direction is fixed, is so formed as to be thicker
than the free layer 101 and serves as a spin filter, i.e. a
mechanism for generating the spin-polarized current. A state in
which the magnetization directions of the free layer 101 and the
pinned layer 103 are parallel to each other is related to data "0",
while a state in which they are anti-parallel to each other is
related to data "1".
The spin transfer magnetization switching shown in FIG. 1 is
achieved by a CPP (Current Perpendicular to Plane) method, where a
write current is injected in a direction perpendicular to the film
plane. More specifically, the current is flowed from the pinned
layer 103 to the free layer 101 in a transition from data "0" to
data "1". In this case, electrons having the same spin state as
that of the pinned layer 103 being the spin filter move from the
free layer 101 to the pinned layer 103. As a result of the spin
transfer (transfer of spin angular momentum) effect, the
magnetization of the free layer 101 is switched. On the other hand,
the current is flowed from the free layer 101 to the pinned layer
103 in a transition from data "1" to data "0". In this case,
electrons having the same spin state as that of the pinned layer
103 being the spin filter move from the pinned layer 103 to the
free layer 101. As a result of the spin transfer effect, the
magnetization of the free layer 101 is switched.
In this manner, the data writing is performed by transferring the
spin electrons in the spin transfer magnetization switching. It is
possible to set the magnetization direction of the free layer 101
depending on the direction of the spin-polarized current
perpendicular to the film plane. Here, it is known that the
threshold value of the writing (magnetization switching) depends on
current density. Therefore, the write current necessary for the
magnetization switching decreases with the reduction of the size of
the memory cell. Since the write current is decreased with the
miniaturization of the memory cell, the spin transfer magnetization
switching is important in realizing a large-capacity MRAM.
As a related technique, U.S. Pat. No. 6,834,005 discloses a
magnetic shift resister that utilizes the spin transfer. The
magnetic shift resister stores data by utilizing a domain wall in a
magnetic body. In the magnetic body having a large number of
separated regions (magnetic domains), a current is so flowed as to
pass through the domain wall and the current causes the domain wall
to move. The magnetization direction in each of the regions is
treated as a record data. For example, such a magnetic shift
resister is used for recording large quantities of serial data. It
should be noted that the domain wall motion in a magnetic body is
reported also in Yamaguchi et al., PRL, Vol. 92, pp. 077205-1,
2004.
Japanese Laid-Open Patent Application JP-2005-191032 discloses a
magnetic storage device provided with a magnetization fixed layer
whose magnetization is fixed, a tunnel insulating layer laminated
on the magnetization fixed layer, and a magnetization free layer
laminated on the tunnel insulating layer. The magnetization free
layer has a connector section overlapping with the tunnel
insulating layer and the magnetization fixed layer, constricted
sections adjacent to both ends of the connector section, and a pair
of magnetization fixed sections respectively formed adjacent to the
constricted sections. The magnetization fixed sections are
respectively provided with fixed magnetizations whose directions
are opposite to each other. The magnetic storage device is further
provided with a pair of magnetic information writing terminals
which is electrically connected to the pair of magnetization fixed
sections. By using the pair of magnetic information writing
terminals, a current penetrating through the connector section, the
pair of constricted sections and the pair of magnetization fixed
sections in the magnetization free layer is flowed.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a new data writing
method for an MRAM.
Another object of the present invention is to provide an MRAM and a
data writing method which can suppress deterioration of a tunnel
barrier layer in an MTJ.
Still another object of the present invention is to provide an MRAM
and a data writing method which can reduce a write current with
reduction of a size of a memory cell.
Still another object of the present invention is to provide an MRAM
and a data writing method which can increase a write speed with
reduction of a size of a memory cell.
In a first aspect of the present invention, a magnetic memory cell
is provided with a magnetic recording layer that is a ferromagnetic
layer and a pinned layer connected to the magnetic recording layer
through a non-magnetic layer. The magnetic recording layer includes
a magnetization switching region, a first magnetization fixed
region and a second magnetization fixed region. The magnetization
switching region has reversible magnetization and is so provided as
to face the pinned layer. The first magnetization fixed region is
connected to a first boundary of the magnetization switching region
and its magnetization direction is fixed to a first direction. The
second magnetization fixed region is connected to a second boundary
of the magnetization switching region and its magnetization
direction is fixed to a second direction. Both of the first
direction and the second direction are toward the magnetization
switching region or away from the magnetization switching region.
For example, the first direction is toward the first boundary, and
the second direction is toward the second boundary. Alternatively,
the first direction is away from the first boundary, and the second
direction is away from the second boundary.
The magnetization direction of the magnetization switching region
is toward any of the first boundary and the second boundary. A
domain wall is formed at any of the first boundary and the second
boundary in the magnetic recording layer. The domain wall in the
magnetic recording layer moves between the first boundary and the
second boundary of the magnetization switching region, due to a
current flowing between the first magnetization fixed region and
the second magnetization fixed region.
For example, the magnetization switching region, the first
magnetization fixed region and the second magnetization fixed
region are formed in the same plane. Preferably, the first
magnetization fixed region and the second magnetization fixed
region are formed parallel to each other, and the magnetization
switching region is so formed as to connect between the first
magnetization fixed region and the second magnetization fixed
region.
For example, the magnetization switching region, the first
magnetization fixed region and the second magnetization fixed
region are linearly formed in the same plane. In this case, the
first direction is opposite to the second direction. Preferably, a
cross-sectional area of the magnetization switching region
increases with distance from the first boundary and the second
boundary, i.e. the magnetization switching region is formed to be
most thick in its central portion. In a plane parallel to the first
boundary and the second boundary, cross-sectional areas of the
first magnetization fixed region and the second magnetization fixed
region may be smaller than a cross-sectional area of the
magnetization switching region.
Also, the first magnetization fixed region and the second
magnetization fixed region may be formed such that the first
direction and the second direction are the same. In this case, for
example, the magnetization switching region, the first
magnetization fixed region and the second magnetization fixed
region are so formed as to have a U-shape. The magnetization
switching region, the first magnetization fixed region and the
second magnetization fixed region may be formed in the same plane.
Alternatively, the magnetization switching region may be formed
parallel to a first plane, while the first magnetization fixed
region and the second magnetization fixed region may be formed
perpendicular to the first plane. For example, the magnetization
switching region is formed on a bottom surface of a trench section,
while the first magnetization fixed region and the second
magnetization fixed region are respectively formed on opposed side
surfaces of the trench section.
The magnetic memory cell can be further provided with a first
magnetic body applying a bias magnetic field of the first direction
to the first magnetization fixed region and a second magnetic body
applying a bias magnetic field of the second direction to the
second magnetization fixed region. The first magnetic body and the
second magnetic body may be so provided as to be in contact with
the first magnetization fixed region and the second magnetization
fixed region, respectively. In this case, the magnetization
direction of the first magnetic body is the first direction and the
magnetization direction of the second magnetic body is the second
direction.
Also, the first magnetic body and the second magnetic body may be
so provided as to be apart from the first magnetization fixed
region and the second magnetization fixed region, respectively. For
example, the first magnetic body and the second magnetic body are
provided above or below the first magnetization fixed region and
the second magnetization fixed region, respectively. In this case,
the magnetization direction of the first magnetic body is opposite
to the first direction and the magnetization direction of the
second magnetic body is opposite to the second direction. Also, the
first magnetic body and the second magnetic body may be provided in
the same plane as that of the magnetic recording layer. Preferably,
the first magnetic body and the second magnetic body are so
provided as to sandwich the magnetic recording layer from both
sides. In this case, the magnetization direction in an end section
of the first magnetic body closest to the first magnetization fixed
region is the first direction, and the magnetization direction in
an end section of the second magnetic body closest to the second
magnetization fixed region is the second direction.
The first magnetization fixed region and the second magnetization
fixed region may have magnetic anisotropy of the same direction,
and the magnetization switching region may have magnetic anisotropy
of a direction different from that of the first magnetization fixed
region and the second magnetization fixed region. A longitudinal
direction of the first magnetization fixed region and a
longitudinal direction of the second magnetization fixed region may
be the same, and a longitudinal direction of the magnetization
switching region may be different from the longitudinal direction
of the first magnetization fixed region and the second
magnetization fixed region. In this case, an external magnetic
field whose direction is the same as the first direction and the
second direction may be applied.
The data writing to the magnetic memory cell is performed in the
following manner. In a first write operation, a first write current
is flowed from the first magnetization fixed region through the
magnetization switching region to the second magnetization fixed
region. On the other hand, in a second write operation, a second
write current is flowed from the second magnetization fixed region
through the magnetization switching region to the first
magnetization fixed region.
The domain wall is formed at the first boundary in the magnetic
recording layer due to the first write operation, while the domain
wall is formed at the second boundary in the magnetic recording
layer due to the second write operation. In a case where the first
direction is toward the first boundary and the second direction is
toward the second boundary, the magnetization of the magnetization
switching region turns toward the first boundary due to the first
write operation, while the magnetization of the magnetization
switching region turns toward the second boundary due to the second
write operation. In a case where the first direction is away from
the first boundary and the second direction is away from the second
boundary, the magnetization of the magnetization switching region
turns toward the second boundary due to the first write operation,
while the magnetization of the magnetization switching region turns
toward the first boundary due to the second write operation.
Also, an assist wiring which intersects with the magnetization
switching region can be provided. Due to a current flowing through
the assist wiring, an assist magnetic field is applied to the
magnetization switching region. The assist wiring is designed such
that the direction of the assist magnetic field assists the
magnetization switching. Preferably, the assist wiring is connected
to the first magnetization fixed region or the second magnetization
fixed region. That is to say, the first write current flows through
the assist wiring in the first write operation, while the second
write current flows through the assist wiring in the second write
operation. The assist wiring may include a first assist wiring
formed below the magnetization switching region and a second assist
wiring formed above the magnetization switching region.
In a read operation, a read current is flowed between the pinned
layer and any of the first magnetization fixed region and the
second magnetization fixed region through the magnetization
switching region and the non-magnetic layer.
In the magnetic memory cell, the magnetic recording layer may
further include another magnetization switching region having
reversible magnetization and a third magnetization fixed region
whose magnetization direction is fixed to a third direction. The
other magnetization switching region is connected to the second
magnetization fixed region at a third boundary and is connected to
the third magnetization fixed region at a fourth boundary. Both of
the second direction and the third direction are toward the other
magnetization switching region or away from the other magnetization
switching region. The first magnetization fixed region, the second
magnetization fixed region and the third magnetization fixed region
are formed parallel to each other. The magnetization switching
region is so formed as to connect between the first magnetization
fixed region and the second magnetization fixed region. The other
magnetization switching region is so formed as to connect between
the second magnetization fixed region and the third magnetization
fixed region. For example, the magnetization switching region, the
other magnetization switching region, the first magnetization fixed
region, the second magnetization fixed region and the third
magnetization fixed region are linearly formed. In this case, the
first direction is opposite to the second direction, and the first
direction and the third direction are the same. Alternatively, the
first magnetization fixed region, the second magnetization fixed
region and the third magnetization fixed region may be formed such
that the first direction, the second direction and the third
direction are the same. The other magnetization switching region is
connected to another pinned layer through another non-magnetic
layer.
In the present invention, it is preferable that the magnetization
switching region is made of soft magnetic material. For example,
the magnetization switching region includes at least one element
selected from the group consisting of Co, Fe and Ni. Preferably,
the soft magnetic material is microcrystalline in which grain
diameter is not more than a film thickness, or amorphous. For
example, composition of the magnetization switching region is
represented by XX-YY-ZZ. In this case, the XX includes at least one
element selected from the group consisting of Co, Fe and Ni. The YY
includes at least one element selected from the group consisting of
Al, Si, Mg, Ta, Nb, Zr, Hf, W, Mo, Ti and V. The ZZ includes at
least one element selected from the group consisting of N, C, B and
O. Also, composition of the magnetization switching region may be
represented by XX-YY. The XX and the YY are as descried above.
Moreover, composition of the magnetization switching region may be
represented by XX-ZZ. The XX and the ZZ are as described above.
In a second aspect of the present invention, a magnetic memory cell
is provided with a magneto resistance element, a first
magnetization fixed section and a second magnetization fixed
section. The magnetoresistance element has a free layer, a pinned
layer, and a non-magnetic layer sandwiched between the free layer
and the pinned layer. The first magnetization fixed section is
connected to a first boundary of the free layer and its
magnetization direction is fixed to a first direction. The second
magnetization fixed section is connected to a second boundary of
the free layer and its magnetization direction is fixed to a second
direction. Both of the first direction and the second direction are
toward the free layer or away from the free layer. In the free
layer, a domain wall moves between the first boundary and the
second boundary, due to a current flowing between the first
magnetization fixed section and the second magnetization fixed
section.
In a third aspect of the present invention, a magnetic random
access memory is provided with the above-described magnetic memory
cell, a word line connected to the magnetic memory cell; and a bit
line connected to the magnetic memory cell.
For example, a first bit line is connected to the first
magnetization fixed region through a first transistor. A second bit
line is connected to the second magnetization fixed region through
a second transistor. The word line is connected to gates of the
first transistor and the second transistor. A write current supply
circuit is connected to the first bit line and the second bit line.
In the first write operation, the word line is selected, and the
write current supply circuit supplies the first write current from
the first bit line to the second bit line through the first
transistor, the magnetic recording layer and the second transistor.
On the other hand, in the second write operation, the word line is
selected, and the write current supply circuit supplies the second
write current from the second bit line to the first bit line
through the second transistor, the magnetic recording layer and the
first transistor.
Also, the second magnetization fixed region of the magnetic memory
cell may be grounded. In this case, the bit line is connected to
the first magnetization fixed region through a transistor, and the
word line is connected to a gate of the transistor. A write current
supply circuit is connected to the bit line. In the first write
operation, the word line is selected, and the write current supply
circuit supplies the first write current from the bit line to the
magnetic memory cell through the transistor. On the other hand, in
the second write operation, the word line is selected, and the
write current supply circuit draws the second write current from
the magnetic memory cell through the transistor and the bit
line.
In a fourth aspect of the present invention, a data read and write
method for a magnetic random access memory is provided. The
magnetic random access memory is provided with the above-described
magnetic memory cell. The data read and write method includes (A) a
step of supplying a first write current from the first
magnetization fixed region through the magnetization switching
region to the second magnetization fixed region, when writing a
first data, and (B) a step of supplying a second write current from
the second magnetization fixed region through the magnetization
switching region to the first magnetization fixed region, when
writing a second data.
In a fifth aspect of the present invention, a data read and write
method for a magnetic random access memory is provided. The
magnetic random access memory is provided with the above-described
magnetic memory cell. The data read and write method includes (A) a
step of moving the domain wall in the magnetic recording layer to
the first boundary by supplying a first write current from the
first magnetization fixed region to the second magnetization fixed
region, when writing a first data, and (B) a step of moving the
domain wall to the second boundary by supplying a second write
current from the second magnetization fixed region to the first
magnetization fixed region, when writing a second data.
The data read and write method further includes (C) a step of
flowing a read current between the pinned layer and any of the
first magnetization fixed region and the second magnetization fixed
region through the magnetization switching region and the
non-magnetic layer, when reading the first data or the second data
stored in the magnetic memory cell.
According to the present invention, a new data writing method for
the MRAM is provided. More specifically, the write current is
flowed not in a direction penetrating through the MTJ but planarly
in the magnetic recording layer. Due to the spin transfer effect by
the spin electrons, the magnetization of the magnetization
switching region in the magnetic recording layer is switched to a
direction depending on the write current direction. The domain wall
in the magnetic recording layer moves back and forth like a
"seesaw" between the first boundary and the second boundary, in
accordance with the moving direction of the electrons of the write
current. That is to say, the domain wall moves within the
magnetization switching region (Domain Wall Motion).
Since the write current does not penetrate through the MTJ at the
time of data writing, deterioration of the tunnel barrier layer in
the MTJ is suppressed. Moreover, since the data writing is achieved
by the spin transfer method, the write current is decreased with
the reduction of the size of the memory cell. Furthermore, since a
moving distance of the domain wall becomes shorter with the
reduction of the size of the memory cell, the write speed is
increased with the miniaturization of the memory cell.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram for explaining data writing according to a
conventional spin transfer method;
FIG. 2 is an overall view showing a structural example of a
magnetic memory cell according to a first exemplary embodiment of
the present invention;
FIG. 3 is a plan view showing the structure of the magnetic memory
cell shown in FIG. 2;
FIG. 4 is a plan view showing a principle of data writing for the
magnetic memory cell shown in FIG. 3;
FIG. 5 is a plan view showing another structural example of a
magnetic memory cell according to the first exemplary embodiment
and a principle of data writing for the magnetic memory cell;
FIG. 6 is a plan view showing still another structural example of a
magnetic memory cell according to the first exemplary
embodiment;
FIG. 7 is a plan view showing a principle of data writing for the
magnetic memory cell shown in FIG. 6;
FIG. 8 is a plan view showing still another structural example of a
magnetic memory cell according to the first exemplary embodiment
and a principle of data writing for the magnetic memory cell;
FIG. 9A is a plan view showing still another structural example of
the magnetic recording layer according to the first exemplary
embodiment;
FIG. 9B is a plan view showing still another structural example of
the magnetic recording layer according to the first exemplary
embodiment;
FIG. 9C is a plan view showing still another structural example of
the magnetic recording layer according to the first exemplary
embodiment;
FIG. 9D is a plan view showing still another structural example of
the magnetic recording layer according to the first exemplary
embodiment;
FIG. 10A is a plan view schematically showing a circuit
configuration of the magnetic memory cell according to the first
exemplary embodiment;
FIG. 10B is a cross-sectional view schematically showing a circuit
configuration of the magnetic memory cell according to the first
exemplary embodiment;
FIG. 11 is a chart showing a summary of the data read/write method
according to the first exemplary embodiment;
FIG. 12 is a circuit block diagram showing an example of a circuit
configuration of an MRAM according to the first exemplary
embodiment;
FIG. 13 is a side view showing an example of a method for fixing
magnetization directions in magnetization fixed regions;
FIG. 14 is a side view showing another example of a method for
fixing magnetization directions in magnetization fixed regions;
FIG. 15 is a side view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 16 is a side view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 17 is a plan view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 18 is a plan view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 19 is a side view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 20 is a plan view showing still another example of a method
for fixing magnetization directions in magnetization fixed
regions;
FIG. 21 is a plan view showing a structural example of a magnetic
memory cell according to a second exemplary embodiment of the
present invention;
FIG. 22 is a plan view showing another structural example of a
magnetic memory cell according to the second exemplary
embodiment;
FIG. 23 is a plan view showing still another structural example of
a magnetic memory cell according to the second exemplary
embodiment;
FIG. 24 is a plan view showing still another structural example of
a magnetic memory cell according to the second exemplary
embodiment;
FIG. 25 is a plan view showing a structural example of a magnetic
memory cell according to a third exemplary embodiment of the
present invention;
FIG. 26 is a plan view showing another structural example of a
magnetic memory cell according to the third exemplary
embodiment;
FIG. 27 is a side view showing a structural example of a magnetic
memory cell according to a fourth exemplary embodiment of the
present invention;
FIG. 28A is a plan view schematically showing a circuit
configuration of a magnetic memory cell according to a fifth
exemplary embodiment of the present invention;
FIG. 28B is a cross-sectional view schematically showing a circuit
configuration of the magnetic memory cell according to the fifth
exemplary embodiment;
FIG. 29 is a cross-sectional view schematically showing a
structural example of a magnetic memory cell according to a sixth
exemplary embodiment of the present invention;
FIG. 30 is a diagram for explaining a method of manufacturing the
magnetic memory cell according to the sixth exemplary
embodiment;
FIG. 31 is an overhead view showing a structural example of a
magnetic recording layer according to a seventh exemplary
embodiment of the present invention;
FIG. 32 is a cross-sectional view schematically showing a
structural example of a magnetic memory cell according to the
seventh exemplary embodiment;
FIG. 33 is an overhead view showing an example of a magnetic memory
cell according to an eighth exemplary embodiment of the present
invention;
FIG. 34 is a plan view showing the magnetic memory cell shown in
FIG. 33;
FIG. 35 is an overhead view showing another example of a magnetic
memory cell according to the eighth exemplary embodiment;
FIG. 36 is an overhead view showing still another example of a
magnetic memory cell according to the eighth exemplary embodiment;
and
FIG. 37 is a plan view showing a structure of an MRAM according to
a ninth exemplary embodiment of the present invention and a
principle of data writing for the MRAM.
BEST MODE FOR CARRYING OUT THE INVENTION
A magnetic memory cell, a magnetic random access memory and a data
read/write method for the magnetic random access memory according
to the present invention will be described below with reference to
the attached drawings.
1. First Exemplary Embodiment
1-1. Structure of Magnetic Memory Cell and Principle of Data
Writing
FIG. 2 shows an example of a magnetic memory cell 1
(magnetoresistance element) according to a first exemplary
embodiment. The magnetic memory cell 1 is provided with a magnetic
recording layer 10 and a pinned layer 30 that are ferromagnetic
layers and a tunnel barrier layer 20 that is a non-magnetic layer.
The tunnel barrier layer 20 is sandwiched between the magnetic
recording layer 10 and the pinned layer 30, and the magnetic
recording layer 10, the tunnel barrier layer 20 and the pinned
layer 30 form an MTJ (magnetic tunnel junction).
The tunnel barrier layer 20 is a thin insulating layer, which is
formed by oxidizing an Al film, for example.
The pinned layer 30 is a laminated film made of CoFe/Ru/CoFe/PtMn
for example, and the magnetization direction thereof is fixed. The
magnetic recording layer 10 plays a role corresponding to a free
layer. The magnetic recording layer 10 is made of soft magnetic
material. The magnetic recording layer 10 includes at least one
element selected from the group consisting of Co, Fe and Ni. For
example, the magnetic recording layer 10 is made of CoFe.
As shown in FIG. 2, the magnetic recording layer 10 according to
the present exemplary embodiment includes three different regions;
a first magnetization fixed region 11, a second magnetization fixed
region 12 and a magnetization switching region 13. The first
magnetization fixed region 11 is so formed as to extend in a Y
direction, and the magnetization direction thereof is fixed.
Similarly, the second magnetization fixed region 12 is so formed as
to extend in the Y direction, and the magnetization direction
thereof is fixed. On the other hand, the magnetization switching
region 13 is so formed as to extend in an X direction and has
reversible magnetization. Also, the magnetization switching region
13 is so formed as to face the pinned layer 30. In other words, a
part of the magnetization switching region 13 of the magnetic
recording layer 10 is connected to the pinned layer 30 through the
tunnel barrier layer 20.
The above-mentioned first magnetization fixed region 11, the second
magnetization fixed region 12 and the magnetization switching
region 13 are formed in the same plane (i.e. XY plane). A shape of
the magnetic recording layer 10 in the XY plane is shown in FIG. 3.
According to the present exemplary embodiment, as shown in FIG. 3,
the first magnetization fixed region 11 and the second
magnetization fixed region 12 are so formed along the Y direction
as to be substantially parallel to each other. The magnetization
switching region 13 is so formed along the X direction as to
connect between the first magnetization fixed region 11 and the
second magnetization fixed region 12. The first magnetization fixed
region 11 and the magnetization switching region 13 are in contact
with each other at a first boundary B1, while the second
magnetization fixed region 12 and the magnetization switching
region 13 are in contact with each other at a second boundary B2.
In the magnetization switching region 13, the first boundary B1 and
the second boundary B2 are at opposed positions. That is to say,
the first and second magnetization fixed regions 11 and 12 and the
magnetization switching region 13 in FIG. 3 are formed to have a
"U-shape or concave shape".
The magnetization direction in each region is also indicated by an
arrow in FIG. 3. Moreover, projection of the pinned layer 30 and
the magnetization direction thereof are also indicated by a dotted
line and a dotted arrow, respectively. Let us consider a case where
the magnetization direction of the pinned layer 30 is fixed to the
-X direction. In FIG. 3, the magnetization direction of the first
magnetization fixed region 11 is fixed to the +Y direction. The
direction is the one away from the first boundary B1. Also, the
magnetization direction of the second magnetization fixed region 12
is fixed to the +Y direction. The direction is the one away from
the second boundary B2. That is to say, the first magnetization
fixed region 11 and the second magnetization fixed region 12 are
both formed such that the magnetization directions thereof are away
from the magnetization switching region 13. This means that the
magnetization direction of the first magnetization fixed region 11
and the magnetization direction of the second magnetization fixed
region 12 are opposite along the shape of the magnetic recording
layer 10. It should be noted that "the fixation of the
magnetization" will be described later (refer to Section 1-3).
On the other hand, the magnetization direction of the magnetization
switching region 13 is reversible, and can be either the +X
direction or the -X direction. In other words, the magnetization of
the magnetization switching region 13 is allowed to be parallel or
anti-parallel to the magnetization of the pinned layer 30. In a
case where the magnetization direction of the magnetization
switching region 13 is the +X direction, namely, the magnetization
is directed toward the second boundary B2, the first magnetization
fixed region 11 forms one magnetic domain while the magnetization
switching region 13 and the second magnetization fixed region 12
form another magnetic domain. In this case, a "domain wall" is
formed at the first boundary B1. On the other hand, in a case where
the magnetization direction of the magnetization switching region
13 is the -X direction, namely, the magnetization is directed
toward the first boundary B1, the first magnetization fixed region
11 and the magnetization switching region 13 form one magnetic
domain while the second magnetization fixed region 12 forms another
magnetic domain. In this case, the domain wall is formed at the
second boundary B2.
As described above, the magnetization of the magnetization
switching region 13 is directed to either the first boundary B1 or
the second boundary B2, and the domain wall is formed at either the
first boundary B1 or the second boundary B2 in the magnetic
recording layer 10. This is because the magnetization direction of
the first magnetization fixed region 11 and the magnetization
direction of the second magnetization fixed region 12 are opposite
along the shape of the magnetic recording layer 10.
Described below will be a principle of data writing with respect to
the magnetic memory cell 1. According to the present exemplary
embodiment, the data writing is achieved by the spin transfer
method (Spin Transfer Data Writing).
FIRST STRUCTURAL EXAMPLE
FIG. 4 shows a principle of data writing for the structure shown in
FIG. 3. A state in which the magnetization directions of the
magnetization switching region 13 and the pinned layer 30 are
parallel to each other is related to data "0". In the data "0"
state, the magnetization direction of the magnetization switching
region 13 is the -X direction, and the domain wall DW exists at the
second boundary B2. On the other hand, a sate in which the
magnetization directions of the magnetization switching region 13
and the pinned layer 30 are anti-parallel to each other is related
to data "1". In the data "1" state, the magnetization direction of
the magnetization switching region 13 is the +X direction, and the
domain wall DW exists at the first boundary B1.
In the present exemplary embodiment, a write current IW is flowed
not in a direction penetrating through the MTJ but planarly in the
magnetic recording layer 10. More specifically, at a time of
writing the data "1" (first write operation), a first write current
IW1 is flowed from the first magnetization fixed region 11 to the
second magnetization fixed region 12 through the magnetization
switching region 13. In this case, electrons (spin electrons) are
injected from the second magnetization fixed region 12 into the
magnetization switching region 13. The spin of the injected
electrons affects a magnetic moment of the magnetization switching
region 13. As a result, the magnetization direction of the
magnetization switching region 13 is switched to a direction toward
the second boundary B2. That is to say, the magnetization of the
magnetization switching region 13 is reversed due to the spin
transfer effect and the magnetization direction thereof is changed
to the +X direction (Spin Transfer Magnetization Switching).
On the other hand, at a time of writing the data "0" (second write
operation), a second write current IW2 is flowed from the second
magnetization fixed region 12 to the first magnetization fixed
region 11 through the magnetization switching region 13. In this
case, electrons are injected from the first magnetization fixed
region 11 into the magnetization switching region 13. As a result,
the magnetization of the magnetization switching region 13 is
reversed and the magnetization direction thereof is changed to the
-X direction. In this manner, according to the present exemplary
embodiment, the magnetization direction of the magnetization
switching region 13 is switched by the write currents IW1 and IW2
which flows planarly in the magnetic recording layer 10. The first
magnetization fixed region 11 and the second magnetization fixed
region 12 serve as supply sources of the electrons having different
spins.
The aforementioned write operation can also be described from a
viewpoint of "domain wall motion". At the time of writing the data
"1", electrons move from the second magnetization fixed region 12
toward the first magnetization fixed region 11. At this time, the
domain wall DW moves from the second boundary B2 to the first
boundary B1 in accordance with the electron moving direction. On
the other hand, at the time of writing the data "0", electrons move
from the first magnetization fixed region 11 toward the second
magnetization fixed region 12. At this time, the domain wall DW
moves from the first boundary B1 to the second boundary B2 in
accordance with the electron moving direction. That is to say, the
domain wall DW in the magnetic recording layer 10 moves back and
forth like a "seesaw or flowmeter" between the first boundary B1
and the second boundary B2 in accordance with the electron moving
direction. Since the domain wall DW moves within the magnetization
switching region 13, the magnetization switching region 13 can be
called a "domain wall moving region". It can also be said that the
magnetic memory cell 1 according to the present exemplary
embodiment stores data on the basis of the position of the domain
wall DW.
It is desirable from a viewpoint of the domain wall motion to
suppress crystal defects in the magnetization switching region 13.
It is therefore preferable that at least the magnetization
switching region 13 is made of an amorphous or microcrystalline
soft magnetic material. Here, microcrystalline means that the grain
diameter is nor more than a film thickness of the magnetization
switching region 13. More specifically, composition of the
magnetization switching region 13 is represented by XX-YY-ZZ. In
this case, the XX includes at least one element selected from the
group consisting of Co, Fe and Ni. The YY includes at least one
element selected from the group consisting of Al, Si, Mg, Ta, Nb,
Zr, Hf, W, Mo, Ti and V. The ZZ includes at least one element
selected from the group consisting of N, C, B and O. Also, the
composition of the magnetization switching region 13 can be
represented by XX-YY. The XX and the YY are as descried above.
Moreover, the composition of the magnetization switching region 13
may be represented by XX-ZZ. The XX and the ZZ are as described
above. Due to such the composition, the magnetization switching
region 13 becomes amorphous or microcrystalline. As a result, the
crystal defects in the magnetization switching region 13 are
suppressed, which makes it possible for the domain wall DW to move
smoothly.
As described above, since the write currents IW1 and IW2 do not
penetrate through the MTJ, deterioration of the tunnel barrier
layer 20 in the MTJ can be suppressed. Moreover, since the data
writing is achieved by the spin transfer method, the write currents
IW1 and IW2 can be decreased with the reduction of the size of the
memory cell. Furthermore, since a moving distance of the domain
wall DW becomes shorter with the reduction of the size of the
memory cell, the write speed can be increased with the
miniaturization of the memory cell.
A data read operation is as follows. At the time of data reading, a
read current is so supplied as to flow between the pinned layer 30
and the magnetization switching region 13. For example, the read
current is flowed from any of the first magnetization fixed region
11 and the second magnetization fixed region 12 to the pinned layer
30 through the magnetization switching region 13 and the tunnel
barrier layer 20. Alternatively, the read current is flowed from
the pinned layer 30 to any of the first magnetization fixed region
11 and the second magnetization fixed region 12 through the tunnel
barrier layer 20 and the magnetization switching region 13. Based
on the read current or a read potential, a resistance value of the
magnetoresistance element is detected and the magnetization
direction of the magnetization switching region 13 is sensed.
SECOND STRUCTURAL EXAMPLE
The magnetization direction of the first magnetization fixed region
11 and the magnetization direction of the second magnetization
fixed region 12 are not limited to those shown in FIGS. 3 and 4.
The magnetization direction of the first magnetization fixed region
11 and the magnetization direction of the second magnetization
fixed region 12 just need to be opposite along the shape of the
magnetic recording layer 10. Another structure according to the
present exemplary embodiment and a principle of data writing with
respect to the structure are shown in FIG. 5. FIG. 5 is a diagram
corresponding to FIG. 4, and an overlapping description will be
omitted as appropriate.
The magnetization direction of the first magnetization fixed region
11 is fixed to the -Y direction. The direction is the one toward
the first boundary B1. Also, the magnetization direction of the
second magnetization fixed region 12 is fixed to the -Y direction.
The direction is the one toward the second boundary B2. That is to
say, the magnetization of the first magnetization fixed region 11
and the magnetization of the second magnetization fixed region 12
are both fixed to a direction toward the magnetization switching
region 13 and are opposite along the shape of the magnetic
recording layer 10. Also, the magnetization direction of the pinned
layer 30 is fixed to the +X direction. In the data "0" state, the
magnetization direction of the magnetization switching region 13 is
the +X direction, and the domain wall DW exists at the second
boundary B2. In the data "1" state, on the other hand, the
magnetization direction of the magnetization switching region 13 is
the -X direction, and the domain wall DW exists at the first
boundary B1.
At a time of writing the data "1" (first write operation), a first
write current IW1 is flowed from the first magnetization fixed
region 11 to the second magnetization fixed region 12 through the
magnetization switching region 13. In this case, electrons are
injected from the second magnetization fixed region 12 into the
magnetization switching region 13. As a result, the magnetization
of the magnetization switching region 13 is reversed, and the
magnetization direction thereof is changed to the -X direction. The
domain wall DW moves from the second boundary B2 to the first
boundary B1 in accordance with the electron moving direction. On
the other hand, at a time of writing the data "0" (second write
operation), a second write current IW2 is flowed from the second
magnetization fixed region 12 to the first magnetization fixed
region 11 through the magnetization switching region 13. In this
case, electrons are injected from the first magnetization fixed
region 11 into the magnetization switching region 13. As a result,
the magnetization of the magnetization switching region 13 is
reversed, and the magnetization direction thereof is changed to the
+X direction. The domain wall DW moves from the first boundary B1
to the second boundary B2 in accordance with the electron moving
direction.
The same effects as in the foregoing first structural example can
also be obtained by the structure shown in FIG. 5. The data reading
is also the same as in the foregoing first structural example.
THIRD STRUCTURAL EXAMPLE
The arrangement of the first magnetization fixed region 11, the
second magnetization fixed region 12 and the magnetization
switching region 13 is not limited to the foregoing ones. Another
example of the shape of the magnetic recording layer 10 in the XY
plane is shown in FIG. 6. In FIG. 6, the first and second
magnetization fixed regions 11 and 12 and the magnetization
switching region 13 are "linearly" formed. That is to say, the
first magnetization fixed region 11 and the second magnetization
fixed region 12 are so formed along the X direction as to be
substantially parallel to each other. The magnetization switching
region 13 is so formed along the X direction as to connect between
the first magnetization fixed region 11 and the second
magnetization fixed region 12.
The magnetization direction of the first magnetization fixed region
11 is fixed to the -X direction. The direction is the one away from
the first boundary B1. Also, the magnetization direction of the
second magnetization fixed region 12 is fixed to the +X direction.
The direction is the one away from the second boundary B2. That is
to say, the magnetization of the first magnetization fixed region
11 and the magnetization of the second magnetization fixed region
12 are both fixed to directions away from the magnetization
switching region 13 and are opposite to each other. It should be
noted that "the fixation of the magnetization" will be described
later (refer to Section 1-3). The magnetization direction of the
magnetization switching region 13 is reversible, and can be either
the +X direction or the -X direction. The magnetization direction
of the pinned layer 30 is fixed to the -X direction.
FIG. 7 shows a principle of data writing for the structure shown in
FIG. 6. A state in which the magnetization directions of the
magnetization switching region 13 and the pinned layer 30 are
parallel to each other is related to data "0". In the data "0"
state, the magnetization direction of the magnetization switching
region 13 is the -X direction, and the domain wall DW exists at the
second boundary B2. On the other hand, a sate in which the
magnetization directions of the magnetization switching region 13
and the pinned layer 30 are anti-parallel to each other is related
to data "1".
In the data "1" state, the magnetization direction of the
magnetization switching region 13 is the +X direction, and the
domain wall DW exists at the first boundary B1.
At a time of writing the data "1" (first write operation), a first
write current IW1 is flowed from the first magnetization fixed
region 11 to the second magnetization fixed region 12 through the
magnetization switching region 13. In this case, electrons are
injected from the second magnetization fixed region 12 into the
magnetization switching region 13. As a result, the magnetization
of the magnetization switching region 13 is reversed, and the
magnetization direction thereof is changed to the +X direction. The
domain wall DW moves from the second boundary B2 to the first
boundary B1 in accordance with the electron moving direction. On
the other hand, at a time of writing the data "0" (second write
operation), a second write current IW2 is flowed from the second
magnetization fixed region 12 to the first magnetization fixed
region 11 through the magnetization switching region 13. In this
case, electrons are injected from the first magnetization fixed
region 11 into the magnetization switching region 13. As a result,
the magnetization of the magnetization switching region 13 is
reversed, and the magnetization direction thereof is changed to the
-X direction. The domain wall DW moves from the first boundary B1
to the second boundary B2 in accordance with the electron moving
direction.
The same effects as in the foregoing first structural example can
also be obtained by the structure shown in FIG. 7. The data reading
is also the same as in the foregoing first structural example.
FOURTH STRUCTURAL EXAMPLE
The magnetization direction of the first magnetization fixed region
11 and the magnetization direction of the second magnetization
fixed region 12 are not limited to those shown in FIGS. 6 and 7.
Another structure according to the present exemplary embodiment and
a principle of data writing with respect to the structure are shown
in FIG. 8. FIG. 8 is a diagram corresponding to FIG. 7, and an
overlapping description will be omitted as appropriate.
The first and second magnetization fixed regions 11 and 12 and the
magnetization switching region 13 are "linearly" formed. The
magnetization direction of the first magnetization fixed region 11
is fixed to the +X direction. The direction is the one toward the
first boundary B1. Also, the magnetization direction of the second
magnetization fixed region 12 is fixed to the -X direction. The
direction is the one toward the second boundary B2. That is to say,
the magnetization of the first magnetization fixed region 11 and
the magnetization of the second magnetization fixed region 12 are
both fixed to directions toward the magnetization switching region
13 and are opposite to each other. Also, the magnetization
direction of the pinned layer 30 is fixed to the +X direction. In
the data "0" state, the magnetization direction of the
magnetization switching region 13 is the +X direction, and the
domain wall DW exists at the second boundary B2. On the other hand,
in the data "1" state, the magnetization direction of the
magnetization switching region 13 is the -X direction, and the
domain wall DW exists at the first boundary B1.
At a time of writing the data "1" (first write operation), a first
write current IW1 is flowed from the first magnetization fixed
region 11 to the second magnetization fixed region 12 through the
magnetization switching region 13. In this case, electrons are
injected from the second magnetization fixed region 12 into the
magnetization switching region 13. As a result, the magnetization
of the magnetization switching region 13 is reversed, and the
magnetization direction thereof is changed to the -X direction. The
domain wall DW moves from the second boundary B2 to the first
boundary B1 in accordance with the electron moving direction. On
the other hand, at a time of writing the data "0" (second write
operation), a second write current IW2 is flowed from the second
magnetization fixed region 12 to the first magnetization fixed
region 11 through the magnetization switching region 13. In this
case, electrons are injected from the first magnetization fixed
region 11 into the magnetization switching region 13. As a result,
the magnetization of the magnetization switching region 13 is
reversed, and the magnetization direction thereof is changed to the
+X direction. The domain wall DW moves from the first boundary B1
to the second boundary B2 in accordance with the electron moving
direction.
The same effects as in the foregoing first structural example can
also be obtained by the structure shown in FIG. 8. The data reading
is also the same as in the foregoing first structural example.
FIFTH STRUCTURAL EXAMPLE
In the case where the magnetic recording layer 10 is formed in a
linear shape, it is desirable to stably hold the domain wall DW at
the boundary B1 or B2. From a viewpoint of energy, the domain wall
becomes more stable as the area thereof becomes smaller. Therefore,
structures such as shown in FIGS. 9A to 9D can be considered as the
structure of the magnetic recording layer 10.
FIGS. 9A to 9C are plan views showing examples of the shape of the
magnetic recording layer 10. In FIGS. 9A to 9C, a width of the
magnetization switching region 13 increases with distance from the
first boundary B1 and the second boundary B2. That is to say, the
magnetization switching region 13 is formed to be widest in its
central portion. The thickness may be largest at the central
portion, instead of the width in the XY plane. The point is that a
"cross-sectional area" of the magnetization switching region 13 in
a plane (YZ plane) parallel to the boundaries B1 and B2 increases
with distance from the boundaries B1 and B2. Consequently, the
domain wall DW is likely to move toward the boundary B1 or the
boundary B2 unless acted upon by an external force. Whereas, the
magnetizations of the first magnetization fixed region 11 and the
second magnetization fixed region 12 are fixed, which prevents the
motion of the domain wall. Therefore, the domain wall becomes
stable at either the first boundary B1 or the second boundary
B2.
In FIG. 9A, cross-sectional areas of the magnetization fixed
regions 11 and 12 are constant and equal to the areas of the
boundaries B1 and B2. In other words, the cross-sectional areas of
the magnetization fixed regions 11 and 12 are smaller than the
cross-sectional area of the magnetization switching region 13. In
FIG. 9B, the cross-sectional areas of the magnetization fixed
regions 11 and 12 become smaller outwards, and the magnetic
recording layer 10 as a whole is formed in a diamond shape. In this
case, it is easy to fabricate the magnetic recording layer 10. In
FIG. 9C, on the other hand, the cross-sectional areas of the
magnetization fixed regions 11 and 12 become larger outwards. In
this case, the stability of the domain wall is further
enhanced.
In FIG. 9D, cross-sectional areas of the magnetization fixed
regions 11, 12 and the magnetization switching region 13 are
roughly constant. However, notches 15 are provided on the lateral
part of the magnetic recording layer 10, and the areas of the
boundaries B1 and B2 are smaller than the other portion. As a
result, the domain wall can be held at the boundaries B1 and B2. It
should be noted in the case of the structure shown in FIG. 9D that
the size of the notch 15 is equal to the minimum feature size at
the minimum and hence an area of the whole of the magnetic
recording layer 10 becomes very large. From a viewpoint of the area
and the manufacturing process, the structures shown in FIGS. 9A to
9C are preferable.
In the magnetic memory cell 1 according to the present exemplary
embodiment, as described above, the magnetic recording layer 10
includes the first magnetization fixed region 11, the second
magnetization fixed region 12 and the magnetization switching
region 13. Such the structure can also be considered as follows.
That is, a "first magnetization fixed section" corresponding to the
first magnetization fixed region 11 and a "second magnetization
fixed section" corresponding to the second magnetization fixed
region 12 are added to a usual magnetoresistance element. The usual
magnetoresistance element is provided with a free layer, a pinned
layer, and a non-magnetic layer sandwiched therebetween. The free
layer, the first magnetization fixed section and the second
magnetization fixed section are formed in the same plane. The first
magnetization fixed section is connected to the first boundary B1
of the free layer, while the second magnetization fixed section is
connected to the second boundary B2 of the free layer. Both of the
magnetization direction (first direction) of the first
magnetization fixed section and the magnetization direction (second
direction) of the second magnetization fixed section are toward the
free layer or away from the free layer. By a write current planarly
flowing between the first magnetization fixed section and the
second magnetization fixed section, the domain wall moves between
the first boundary B1 and the second boundary B2, and the
magnetization of the free layer is switched.
1-2. Circuit Configuration
Next, a circuit configuration for supplying the write currents IW1
and IW2 to the magnetic memory cell 1 according to the present
exemplary embodiment will be explained below. FIG. 10A is a plan
view showing an example of a circuit configuration of the magnetic
memory cell 1. Also, FIG. 10B is a cross-sectional view
schematically showing a structure of the magnetic memory cell 1
shown in FIG. 10A.
The first magnetization fixed region 11 of the magnetic recording
layer 10 is connected to a first lower electrode 41 via a through
hall 45, and the second magnetization fixed region 12 is connected
to a second lower electrode 42 via a through hall 46. The first
lower electrode 41 is connected to one of source/drain of a first
transistor TR1, and the other of source/drain of the first
transistor TR1 is connected to a first bit line BL1. Also, the
second lower electrode 42 is connected to one of source/drain of a
second transistor TR2, and the other of source/drain of the second
transistor TR2 is connected to a second bit line BL2. A gate of the
first transistor TR1 and a gate of the second transistor TR2 are
connected to a word line WL.
The pinned layer 30 is formed on the magnetization switching region
13 of the magnetic recording layer 10 through the tunnel barrier
layer 20. An upper electrode 43 is formed on the pinned layer 30,
and a read line 44 is connected to the upper electrode 43. A
direction of the read line 44 is arbitrary. The read line 44 can be
connected to a selection transistor or the ground.
FIG. 11 shows a summary of the data read and write method in the
case of the circuit configuration shown in FIG. 10A and FIG. 10B.
In both of the writing and reading, a word line WL connected to a
target memory cell is selected and its potential is set to "High".
Consequently, the first transistor TR1 and the second transistor
TR2 are turned ON.
In the case of the data "1" writing, potentials of the first bit
line BL1 and the second bit line BL2 are set to "High" and "Low",
respectively. As a result, the first write current IW1 flows from
the first bit line BL1 to the second bit line BL2 through the first
transistor TR1, the magnetic recording layer 10 and the second
transistor TR2. In the case of the data "0" writing, on the other
hand, potentials of the first bit line BL1 and the second bit line
BL2 are set to "Low" and "High", respectively. As a result, the
second write current IW2 flows from the second bit line BL2 to the
first bit line BL1 through the second transistor TR2, the magnetic
recording layer 10 and the first transistor TR1.
At the time of data reading, for example, the potential of the
first bit line BL1 is set to "High" while the second bit line BL2
is set to "Open". Consequently, the read current flows from the
first bit line BL1 to the read line 44 through the first
transistors TR1 and the MTJ. Alternatively, the first bit line BL1
is set to "Open" while the potential of the second bit line BL2 is
set to "High". Consequently, the read current flows from the second
bit line BL2 to the read line 44 through the second transistors TR2
and the MTJ.
Peripheral circuits for controlling the above-mentioned word line
WL, the first bit line BL1 and the second bit line BL2 can be
designed appropriately by a person skilled in the art. One example
of a configuration of the peripheral circuits is shown in FIG.
12.
In FIG. 12, an MRAM 50 has a memory cell array 51 in which the
above-described magnetic memory cells 1 are arranged in a matrix
form. The memory cell array 51 includes not only the magnetic
memory cells 1 used for the data recording but also reference cells
1r which are referred to at the time of data reading. A basic
structure of the reference cell 1r is the same as that of the
magnetic memory cell 1. Let us consider a case where the
above-mentioned read line 44 is connected to the ground line in
each magnetic memory cell 1. Also, one word line and a pair of bit
lines (the first bit line BL1 and the second bit line BL2) are
provided for each magnetic memory cell 1, as described above.
A plurality of word lines WL are connected to an X selector 52. In
any of the data writing and reading, the X selector 52 selects one
word line WL connected to a target memory cell is as a selected
word line WLs out of the plurality of word lines WL.
A plurality of first bit lines BL1 are connected to a Y-side
current termination circuit 54, and a plurality of second bit lines
BL2 are connected to a Y selector 53. In the data writing, the Y
selector 53 selects one second bit line BL2 connected to the target
memory cell is as a selected second bit line BL2s out of the
plurality of second bit lines BL2. In the data writing, the Y-side
current termination circuit 54 selects one first bit line BL1
connected to the target memory cell is as a selected first bit line
BL1s out of the plurality of first bit lines BL1. In this manner,
the target memory cell 1s is selected.
A Y-side current source circuit 55 is a current source for
supplying or drawing a predetermined write current (IW1, IW2) with
respect to the selected second bit line BL2s at the time of data
writing. The Y-side current source circuit 55 includes a current
selector unit for determining the direction of the write current
and a constant current source for supplying a constant current. A
Y-side power source circuit 56 supplies a predetermined voltage to
the Y-side current termination circuit 54 at the time of data
writing. As a result, the write current IW1 or IW2 by the Y-side
current source circuit 55 flows into the Y selector 53 or flows out
from the Y selector 53, depending on the data to be written to the
target memory cell 1s. The above-mentioned X selector 52, Y
selector 53, Y-side current termination circuit 54, Y-side current
source circuit 55 and Y-side power source circuit 56 constitute a
"write current supply circuit" for supplying the write currents IW1
and 1W2 to the magnetic memory cells 1.
At the time of data reading, the first bit lines BL1 are set to
"Open". At the time of data reading, a read current load circuit 57
supplies a predetermined read current to the selected second bit
line BL2s. Also, the read current load circuit 57 supplies the
predetermined current to a reference second bit line BL2r which is
connected to the reference cells 1r. A sense amplifier 58 reads
data from the target memory cell is based on a difference between a
potential of the reference second bit line BL2r and a potential of
the selected second bit line BL2s, and outputs the read data.
1-3. Magnetization Fixation
Next, methods for fixing the magnetizations of the first
magnetization fixed region 11 and the second magnetization fixed
region 12 will be described below. The methods for the
magnetization fixation include three patterns respectively
utilizing exchange coupling, magnetostatic coupling and magnetic
anisotropy.
(Exchange Coupling)
Description will be given by taking the "third structural example"
shown in FIG. 6 as an example. FIG. 13 is a side view schematically
showing the magnetic memory cell 1 provided with a magnetization
fixing means. The magnetic memory cell 1 is provided with a first
magnetic body 61 and a second magnetic body 62 which serve as the
magnetization fixing means. The first magnetic body 61 applies a
bias magnetic field of the -X direction to the first magnetization
fixed region 11. On the other hand, the second magnetic body 62
applies a bias magnetic field of the +X direction to the second
magnetization fixed region 12.
More specifically, the first magnetic body 61 includes a
ferromagnetic layer having magnetization of the -X direction, and
the ferromagnetic layer is formed to be in contact with the first
magnetization fixed region 11. The first magnetic body 61 fixes the
magnetization direction of the first magnetization fixed region 11
to the -X direction by the "exchange coupling". On the other hand,
the second magnetic body 62 includes a ferromagnetic layer having
magnetization of the +X direction, and the ferromagnetic layer is
formed to be in contact with the second magnetization fixed region
12. The second magnetic body 62 fixes the magnetization direction
of the second magnetization fixed region 12 to the +X direction by
the exchange coupling.
As shown in FIG. 13, the first magnetic body 61 is a laminated film
of CoFe/PtMn, for example. The laminated film structure is the one
such as used in the pinned layer. As in the case of the pinned
layer whose magnetization direction is fixed, the magnetization
direction of the CoFe layer which serves as a source for fixing the
magnetization direction of the first magnetization fixed region 11
is firmly fixed to the -X direction. Also, the second magnetic body
62 is a laminated film of CoFe/Ru/CoFe/PtMn, for example. The upper
half of the structure is the same as the structure of the first
magnetic body 61, and the magnetization direction of the CoFe layer
is fixed to the -X direction. The lower CoFe layer is
anti-ferromagnetically coupled with the upper CoFe layer through
the Ru layer, and the magnetization direction thereof is fixed to
the +X direction. The CoFe layer having the magnetization of the +X
direction is contact with the second magnetization fixed region
12.
In FIG. 13, the film structures of the first magnetic body 61 and
the second magnetic body 62 are different from each other, as
described above. The reason is that it is necessary to apply the
bias magnetic fields of opposite directions respectively to the
first magnetization fixed region 11 and the second magnetization
fixed region 12. Alternatively, the first magnetic body 61 and the
second magnetic body 62 may be made of different materials, instead
of the different film structures. Also, a similar first magnetic
body 61 and a similar second magnetic body 62 can be applied to the
"fourth structural example" and the "fifth structural example".
The magnetization fixation based on the exchange coupling can also
be applied to the first structural example shown in FIG. 3. In this
case, the magnetizations of the first magnetization fixed region 11
and the second magnetization fixed region 12 are fixed to the same
+Y direction. Therefore, the first magnetic body 61 and the second
magnetic body 62 can have the same film structure. For example, the
first magnetic body 61 and the second magnetic body 62 are
laminated films of CoFe/PtMn.
Referring to FIG. 14, the first magnetic body 61 and the second
magnetic body 62 apply bias magnetic fields of the +Y direction to
the first magnetization fixed region 11 and the second
magnetization fixed region 12, respectively. More specifically,
each of the first magnetic body 61 and the second magnetic body 62
includes a ferromagnetic layer (CoFe layer) having magnetization of
the +Y direction, and the respective ferromagnetic layers are so
formed as to be in contact with the first magnetization fixed
region 11 and the second magnetization fixed region 12. The first
magnetic body 61 and the second magnetic body 62 respectively fix
the magnetization directions of the first magnetization fixed
region 11 and the second magnetization fixed region 12 to the +Y
direction by the exchange coupling. Also, a similar first magnetic
body 61 and a similar second magnetic body 62 can be applied to the
"second structural example" shown in FIG. 5.
(Magnetostatic Coupling)
Description will be given by taking the "third structural example"
shown in FIG. 6 as an example. FIG. 15 is a side view schematically
showing the magnetic memory cell 1 provided with a magnetization
fixing means. The magnetic memory cell 1 is provided with a first
magnetic body 61 and a second magnetic body 62 which serve as the
magnetization fixing means. The first magnetic body 61 applies a
bias magnetic field of the -X direction to the first magnetization
fixed region 11. On the other hand, the second magnetic body 62
applies a bias magnetic field of the +X direction to the second
magnetization fixed region 12.
More specifically, the first magnetic body 61 includes a
ferromagnetic layer having magnetization of the +X direction
opposite to the -X direction, and the ferromagnetic layer is formed
apart from the first magnetization fixed region 11. The first
magnetic body 61 fixes the magnetization direction of the first
magnetization fixed region 11 to the -X direction by the
"magnetostatic coupling". On the other hand, the second magnetic
body 62 includes a ferromagnetic layer having magnetization of the
-X direction opposite to the +X direction, and the ferromagnetic
layer is formed apart from the second magnetization fixed region
12. The second magnetic body 62 fixes the magnetization direction
of the second magnetization fixed region 12 to the +X direction by
the magnetostatic coupling. In FIG. 15, the first magnetic body 61
and the second magnetic body 62 are provided above the first
magnetization fixed region 11 and the second magnetization fixed
region 12, respectively. The first magnetic body 61 and the second
magnetic body 62 may be provided below or lateral to the first
magnetization fixed region 11 and the second magnetization fixed
region 12, respectively.
As shown in FIG. 15, the first magnetic body 61 is a laminated film
of CoFe/Ru/CoFe/PtMn, for example. The second magnetic body 62 is a
laminated film of CoFe/PtMn, for example. The reason why the first
magnetic body 61 and the second magnetic body 62 have different
film structures is the same as in the case of the exchange
coupling. Also, a similar first magnetic body 61 and a similar
second magnetic body 62 can be applied to the "fourth structural
example" and the "fifth structural example".
FIG. 16 shows a configuration for collectively fixing the
magnetizations with respect to two-bits magnetic memory cells. As
shown in FIG. 16, for example, the first magnetic body 61 is
provided at a position over a gap between the two adjacent magnetic
memory cells. The first magnetic body 61 fixes the magnetization
direction of the first magnetization fixed region 11 of a magnetic
memory cell to the -X direction by the magnetostatic coupling. At
the same time, the first magnetic body 61 fixes the magnetization
direction of the second magnetization fixed region 12 of the
adjacent magnetic memory cell to the -X direction by the
magnetostatic coupling. The magnetization direction of the first
magnetization fixed region 11 (not shown) of the adjacent magnetic
memory cell is fixed to the +X direction by the second magnetic
body 62.
Moreover, as shown in FIGS. 17 and 18, the first magnetic body 61
and the second magnetic body 62 may be provided in the same plane
(XY plane) as that of the magnetic recording layer 10. In FIG. 17,
the first magnetic body 61 and the second magnetic body 62 are
respectively provided near the both ends in the X direction of the
magnetic recording layer 10. The magnetizations of the first
magnetic body 61 and the second magnetic body 62 are directed to
the same direction. Due to such a configuration, the magnetization
of the first magnetization fixed region 11 is fixed to the +X
direction and the magnetization of the second magnetization fixed
region 12 is fixed to the -X direction.
In FIG. 18, the first magnetic body 61 and the second magnetic body
62 are bent and are so formed as to sandwich the magnetic recording
layer 10 from both sides. The magnetization direction in an end
section of the first magnetic body 61 closest to the first
magnetization fixed region 11 is the +X direction. On the other
hand, the magnetization direction in an end section of the second
magnetic body 62 closest to the second magnetization fixed region
12 is the -X direction. That is to say, the magnetization
directions in the end sections are consistent with the
magnetization directions to be fixed in the magnetization fixed
regions 11 and 12, respectively. Such a configuration makes it easy
to achieve the statistic coupling.
It should be noted that the notches 15 may be provided at the
boundaries between the magnetization switching region 13 and the
magnetization fixed regions 11 and 12, as shown in FIGS. 17 and 18.
Alternatively, the magnetic recording layer 10 may have the
structures shown in FIGS. 9A to 9C. In this case, the domain wall
DW is stabilized. Also, the first magnetic body 61 and the second
magnetic body 62 need not be provided in the completely same plane
as that of the magnetic recording layer 10 and can be vertically
displaced.
The magnetization fixation based on the magnetostatic coupling can
also be applied to the first structural example shown in FIG. 3. In
this case, the magnetizations of the first magnetization fixed
region 11 and the second magnetization fixed region 12 are fixed to
the same +Y direction. Therefore, the first magnetic body 61 and
the second magnetic body 62 can have the same film structure. For
example, the first magnetic body 61 and the second magnetic body 62
are laminated films of CoFe/PtMn.
Referring to FIG. 19, the first magnetic body 61 and the second
magnetic body 62 apply bias magnetic fields of the +Y direction to
the first magnetization fixed region 11 and the second
magnetization fixed region 12, respectively. More specifically,
each of the first magnetic body 61 and the second magnetic body 62
includes a ferromagnetic layer (CoFe layer) having magnetization of
the -Y direction opposite to the +Y direction, and the respective
ferromagnetic layers are so formed as to be apart from the first
magnetization fixed region 11 and the second magnetization fixed
region 12. The first magnetic body 61 and the second magnetic body
62 respectively fix the magnetization directions of the first
magnetization fixed region 11 and the second magnetization fixed
region 12 to the +Y direction by the magnetostatic coupling. Also,
a similar first magnetic body 61 and a similar second magnetic body
62 can be applied to the "second structural example" shown in FIG.
5.
(Magnetic Anisotropy)
As to the "first structural example and second structural example"
shown in FIGS. 3 to 5, the exchange coupling or the magnetostatic
coupling is not necessarily applied to. In FIGS. 3 to 5,
longitudinal directions of the first magnetization fixed region 11
and the second magnetization fixed region 12 are the Y direction,
while a longitudinal direction of the magnetization switching
region 13 is the X direction. Therefore, the first magnetization
fixed region 11 and the second magnetization fixed region 12 have
magnetic anisotropy of the same direction, while the magnetization
switching region 13 has magnetic anisotropy of a direction
different from that of the magnetization fixed regions 11 and
12.
Therefore, the magnetization directions of the first magnetization
fixed region 11 and the second magnetization fixed region 12 can be
set to the +Y direction of the -Y direction in an initial annealing
process. The magnetization directions of the first magnetization
fixed region 11 and the second magnetization fixed region 12 are
maintained to the +Y direction or the -Y direction by the magnetic
anisotropy. In this case, the magnetization fixing means such as
the first magnetic body 61 and the second magnetic body 62 is not
necessary, which is preferable from a viewpoint of decrease in the
components. In other words, the "U-shape" shown in FIGS. 3 to 5 is
a preferable shape from a viewpoint of the magnetization
fixation.
Also, an external magnetic field whose direction is the same as the
direction of the fixed magnetization may be uniformly applied to
the magnetic memory cell 1, as shown in FIG. 20. For example, a
magnet of several Oe is provided on a package. As a result, the
magnetization fixation is stabilized and hence thermal stability is
improved. Needless to say, the above-described exchange coupling or
magnetostatic coupling can be applied in addition. In that case,
the magnetization fixation is further stabilized.
1-4. Effects
According to the present invention, as described above, a new data
read and write method is proposed with regard to the
random-accessible MRAM. The data writing is achieved by the domain
wall motion due to the spin transfer within the magnetic recording
layer 10. The data reading is achieved by using the MTJ. Resultant
effects are as follows.
First, excellent selectivity of the memory cell can be ensured as
compared with the asteroid method. In the case of the asteroid
method, variation in a threshold value of a write magnetic field
deteriorates the memory cell selectivity in a 2-dimensional memory
cell array. According to the spin transfer method, however, the
write current acts only on a target memory cell. Thus, the
disturbance is greatly reduced. In other words, a selective writing
property is improved.
Also, a scaling property of the write current is improved as
compared with the asteroid method and the toggle method. In the
case of the asteroid method and the toggle method, a magnetic
switching field necessary for switching the magnetization of the
magnetic recording layer increases in substantially inverse
proportion to the size of the memory cell. That is to say, the
write current tends to increase with the miniaturization of the
memory cell. According to the spin transfer method, however, the
threshold value of the magnetization switching depends on current
density. Since the current density is increased with the reduction
of the size of the memory cell, it is possible to reduce the write
current with the miniaturization of the memory cell. In other
words, it is not necessary to increase the write current when the
size of the memory cell is reduced. In that sense, the scaling
property of the write current is improved. This is important in
realizing a large-capacity MRAM.
Also, a current-magnetic field conversion efficiency is increased
as compared with the asteroid method and the toggle method. In the
case of the asteroid method and the toggle method, the write
current is consumed by Joule heating. It has been necessary to
provide a write-dedicated wiring such as a flux keeper and a yoke
structure in order to enhance the current-magnetic field conversion
efficiency. This causes complexity of the manufacturing process and
an increase in wiring inductance. According to the spin transfer
method, however, the write current directly contributes to the spin
transfer. Therefore, the current-magnetic field conversion
efficiency is increased. Consequently, the complexity of the
manufacturing process and the increase in the wiring inductance can
be prevented.
Moreover, the deterioration of the MTJ (tunnel barrier layer 20) is
suppressed as compared with the conventional spin transfer
magnetization switching. The conventional spin transfer
magnetization switching is achieved by the CPP (Current
Perpendicular to Plane) method, where the write current is injected
in a direction perpendicular to the film plane. The write current
at the time of data writing is much larger than the read current,
and there is a possibility that the large current destroys the
tunnel barrier layer 20. According to the write method in the
present invention, however, a current path at the time of reading
and a current path at the time of writing are separated from each
other. More specifically, the write currents IW1 and 1W2 do not
penetrate through the MTJ but flow within the plane of the magnetic
recording layer 10 at the time of data writing. It is not necessary
at the time of data writing to inject a large current in a
direction perpendicular to the MTJ film plane. Consequently, the
deterioration of the tunnel barrier layer 20 in the MTJ can be
suppressed.
Furthermore, the write speed is increased with the miniaturization
of the memory cell. The reason is that the data writing in the
present invention is achieved by the domain wall motion within the
magnetic recording layer 10. The reduction of the memory cell size
means that a moving distance of the domain wall DW becomes shorter.
Therefore, the write speed is increased with the reduction of the
memory cell size.
According to the present invention, the above-described effects can
be simultaneously obtained. The technique according to the present
invention is extremely useful in order to realize a
high-integration, high-speed operation and low power consumption
MRAM.
2. Second Exemplary Embodiment
Still other patterns can be considered as a shape of the magnetic
recording layer 10. FIGS. 21 to 24 are plan views showing various
patterns of the shape of the magnetic recording layer 10. An end
portion or a corner portion of the magnetic recording layer 10 may
have a round shape instead of a sharp shape. In FIG. 21, for
example, an end portion R1 opposed to the first boundary B1 of the
first magnetization fixed region 11 is rounded. An end portion R1
opposed to the second boundary B2 of the second magnetization fixed
region 12 is rounded. In FIG. 22, an outer corner portion R2
associated with the position of the first boundary B1 and an outer
corner portion R2 associated with the position of the second
boundary B2 are also rounded in addition to the end portions R1. In
FIG. 23, inner corner portions R3 as well as the outer corner
portions R2 are rounded. In FIG. 24, all the end portions and the
corner portions are rounded. Moreover, the magnetization switching
region 13 shown in FIG. 24 does not have a linear section and is
gently curved as a whole. The same effects as in the first
exemplary embodiment can also be obtained by these shapes. The
magnetization switching region 13 just needs to be formed so as to
connect between the first magnetization fixed region 11 and the
second magnetization fixed region 12.
3. Third Exemplary Embodiment
FIGS. 25 and 26 are plan views showing still other structural
examples of the magnetic memory cell. In FIGS. 25 and 26, 2-bit
memory cells have a continuous structure. The magnetic recording
layer 10 includes a first magnetization fixed region 11-1, a first
magnetization fixed region 13-1, a second magnetization fixed
region 12, a second magnetization switching region 13-2 and a third
magnetization fixed region 11-2. The first magnetization switching
region 13-1 and the second magnetization switching region 13-2 are
connected to pinned layers (not shown) through tunnel barrier
layers, respectively.
The first magnetization fixed region 11-1 and the first
magnetization switching region 13-1 are connected to each other at
a first boundary B1, and the first magnetization switching region
13-1 and the second magnetization fixed region 12 are connected to
each other at a second boundary B2. The first boundary B1 and the
second boundary B2 are located at respective ends of the first
magnetization switching region 13-1. Also, the second magnetization
fixed region 12 and the second magnetization switching region 13-2
are connected to each other at a third boundary B3, and the second
magnetization switching region 13-2 and the third magnetization
fixed region 11-2 are connected to each other at a fourth boundary
B4. The third boundary B3 and the fourth boundary B4 are located at
respective ends of the second magnetization switching region
13-2.
In FIG. 25, the first magnetization fixed region 11-1, the second
magnetization fixed region 12 and the third magnetization fixed
region 11-2 are so formed along the Y direction as to be
substantially parallel to each other. The first magnetization
switching region 13-1 is so formed along the X direction as to
connect between the first magnetization fixed region 11-1 and the
second magnetization fixed region 12. The second magnetization
switching region 13-2 is so formed along the X direction as to
connect between the second magnetization fixed region 12 and the
third magnetization fixed region 11-2.
The magnetizations of the first magnetization switching region 13-1
and the second magnetization switching region 13-2 are reversible
and allowed to be directed to either the +X direction or the -X
direction. The magnetization directions of the first magnetization
fixed region 11-1, the second magnetization fixed region 12 and the
third magnetization fixed region 11-2 are fixed to the +Y
direction. In this case, the magnetizations of the first
magnetization fixed region 11-1 and the second magnetization fixed
region 12 are both fixed to directions toward the first
magnetization switching region 13-1. Also, the magnetizations of
the second magnetization fixed region 12 and the third
magnetization fixed region 11-2 are both fixed to directions away
from the second magnetization switching region 13-2. That is to
say, the magnetization of the first magnetization fixed region
11-1, the magnetization of the second magnetization fixed region 12
and the magnetization of the third magnetization fixed region 11-2
are reversed alternately along the shape of the magnetic recording
layer 10.
In FIG. 26, the first magnetization fixed region 11-1, the second
magnetization fixed region 12 and the third magnetization fixed
region 11-2 are so formed along the X direction as to be
substantially parallel to each other. The first magnetization
switching region 13-1 is so formed along the X direction as to
connect between the first magnetization fixed region 11-1 and the
second magnetization fixed region 12. The second magnetization
switching region 13-2 is so formed along the X direction as to
connect between the second magnetization fixed region 12 and the
third magnetization fixed region 11-2. That is to say, the first
magnetization fixed region 11-1, the first magnetization switching
region 13-1, the second magnetization fixed region 12, the second
magnetization switching region 13-2 and the third magnetization
fixed region 11-2 are linearly formed along the X direction.
The magnetizations of the first magnetization switching region 13-1
and the second magnetization switching region 13-2 are reversible
and allowed to be directed to either the +X direction or the -X
direction. The magnetization directions of the first magnetization
fixed region 11-1 and the third magnetization fixed region 11-2 are
fixed to the -X direction, while the magnetization direction of the
second magnetization fixed region 12 is fixed to the +X direction.
In this case, the magnetizations of the first magnetization fixed
region 11-1 and the second magnetization fixed region 12 are both
fixed to directions away from the first magnetization switching
region 13-1. Also, the magnetizations of the second magnetization
fixed region 12 and the third magnetization fixed region 11-2 are
both fixed to directions toward the second magnetization switching
region 13-2. That is to say, the magnetization of the first
magnetization fixed region 11-1, the magnetization of the second
magnetization fixed region 12 and the magnetization of the third
magnetization fixed region 11-2 are reversed alternately along the
shape of the magnetic recording layer 10.
In FIGS. 25 and 26, the first magnetization fixed region 11-1 is
connected to a first bit line BL1 through a first transistor TR1.
The second magnetization fixed region 12 is connected to a second
bit line BL2 through a second transistor TR2. The third
magnetization fixed region 11-2 is connected to a third bit line
BL3 through a third transistor TR3. When a data is written to the
first magnetization switching region 13-1, for example, the first
transistor TR1 and the second transistor TR2 are turned ON and a
write current whose direction is dependent on the write data is
supplied to the first bit line BL1 and the second bit line BL2.
Also, when a data is written to the second magnetization switching
region 13-2, the second transistor TR2 and the third transistor TR3
are turned ON and a write current whose direction is dependent on
the write data is supplied to the second bit line BL2 and the third
bit line BL3. The data reading can be realized by a cross-point
method, for example. The same effects as in the first exemplary
embodiment can also be obtained by these structures.
The generalization of a structure including continuous n-bit memory
cells (n is a natural number) gives the following expression. The
magnetic recording layer includes n magnetization switching regions
A.sub.1 to A.sub.n and n+1 magnetization fixed regions B.sub.1 to
B.sub.n+1. The n magnetization switching regions A.sub.1 to A.sub.n
and the n+1 magnetization fixed regions B.sub.1 to B.sub.n+1 are
arranged alternately. That is to say, the i-th magnetization
switching region A.sub.i (i is an integer not less than 1 and not
more than n) is so formed as to connect between the i-th
magnetization fixed region B.sub.i and the (i+1)-th magnetization
fixed region B.sub.i+1. The magnetization of the i-th magnetization
fixed region B.sub.i and the magnetization of the (i+1)-th
magnetization fixed region B.sub.i+1 are fixed to directions toward
or away from the i-th magnetization switching region A.sub.i. The
magnetizations of adjacent magnetization fixed regions are fixed to
the opposite directions. That is, the magnetizations of the n+1
magnetization fixed regions B.sub.1 to B.sub.n+1 are reversed
alternately along the shape of the magnetic recording layer. Also,
n MTJs are formed with respect to the n magnetization switching
regions A.sub.1 to A.sub.n, respectively. Moreover, the n+1
magnetization fixed regions B.sub.1 to B.sub.n+1 are connected to
n+1 bit lines BL.sub.1 to BL.sub.n+1 through n+1 transistors,
respectively. When a data is written to the i-th magnetization
switching region A.sub.i, a write current whose direction is
dependent on the write data is supplied to the i-th bit line
BL.sub.i and the (i+1)-th bit line BL.sub.i+1.
4. Fourth Exemplary Embodiment
FIG. 27 is a side view showing a structural example of the magnetic
memory cell according to a fourth exemplary embodiment. In the
present exemplary embodiment, a magnetic recording layer 10' is
constituted by an SAF (Synthetic Anti-Ferromagnetic) layer. More
specifically, the magnetic recording layer 10' includes a first
ferromagnetic layer 10a and a second ferromagnetic layer 10b which
are anti-ferromagnetically coupled through an intermediate layer
14. The intermediate layer is a non-magnetic layer, e.g. a Ru
layer. The first ferromagnetic layer 10a includes the first
magnetization fixed region 11a, the second magnetization fixed
region 12a and the magnetization fixed region 13a sandwiched
between the first and second magnetization fixed regions 11a and
12a. Also, the second ferromagnetic layer 10b includes the first
magnetization fixed region 11b, the second magnetization fixed
region 12b and the magnetization fixed region 13b sandwiched
between the first and second magnetization fixed regions 11b and
12b.
The magnetization directions of the first magnetization fixed
regions 11a and 11b are opposite to each other. The magnetization
directions of the second magnetization fixed regions 12a and 12b
are opposite to each other. The magnetization directions of the
magnetization switching regions 13a and 13b are opposite to each
other. The magnetizations of the magnetization switching regions
13a and 13b are reversible and directed to either the +X direction
or the -X direction. When the magnetization of one of the
magnetization switching regions 13a and 13b is reversed, the
magnetization of the other is also reversed. The magnetization
switching region 13a of the first ferromagnetic layer 10a is
adjacent to the pinned layer 30 through the tunnel barrier layer
20. Shown in FIG. 27 is the "0 state" in which the magnetization of
the magnetization switching region 13a and the magnetization of the
pinned layer 30 are parallel to each other. In this case, the
domain wall DW exists at the second boundary B2.
The data writing is performed in the same manner as in the
foregoing exemplary embodiments. At the time of writing data "1",
for example, a write current is flowed from the first magnetization
fixed regions 11a and 11b to the second magnetization fixed regions
12a and 12b in the magnetic recording layer 10'. As a result, both
the magnetizations of the magnetization switching regions 13a and
13b are reversed and the domain wall DW moves to the first boundary
B1. The data reading is performed by sensing the magnetization
direction of the magnetization switching region 13a of the first
ferromagnetic layer 10a with the use of the pinned layer 30. The
same effects as in the first exemplary embodiment can also be
obtained by such a structure. Furthermore, it is expected that an
influence of an external magnetic field is reduced due to the SAF
layer.
5. Fifth Exemplary Embodiment
Although a circuit configuration of the magnetic memory cell which
has the two transistors TR1 and TR2 is shown in FIGS. 10A and 10B,
a circuit configuration is not limited thereto. FIG. 28A is a plan
view showing a circuit configuration of the magnetic memory cell
which has only one transistor TR. Also, FIG. 28B is a
cross-sectional view schematically showing a structure of the
magnetic memory cell shown in FIG. 28A.
The first magnetization fixed region 11 of the magnetic recording
layer 10 is connected to the first lower electrode 41 via the
through hall 45, while the second magnetization fixed region 12 is
connected to the second lower electrode 42 via the through hall 46.
The first lower electrode 41 is connected to one of source/drain of
the transistor TR, and the other of the source/drain of the
transistor TR is connected to a bit line BL. Also, the second lower
electrode 42 is connected to the ground. A gate of the transistor
TR is connected to the word line WL.
At the time of data writing, a word line WL connected to a target
memory cell is selected, and the transistor TR of the target memory
cell is turned ON. The direction of the write current flowed in the
bit line BL is changed depending on the write data. At the time of
writing data "1", for example, the write current supply circuit
supplies the first write current IW1 to the bit line BL. In this
case, the first write current IW1 flows from the bit line BL into
the ground through the transistor TR, the first magnetization fixed
region 11, the magnetization switching region 13 and the second
magnetization fixed region 12. On the other hand, at the time of
writing data "0", the write current supply circuit draws the second
write current IW2 from the ground. In this case, the second write
current IW2 flows from the ground into the bit line BL through the
second magnetization fixed region 12, the magnetization switching
region 13, the first magnetization fixed region 11 and the
transistor TR. The data reading can be realized by a cross-point
method, for example. The same effects as in the first exemplary
embodiment can also be obtained by such a structure.
6. Sixth Exemplary Embodiment
FIG. 29 shows a structural example of a magnetic memory cell
according to a sixth exemplary embodiment of the present invention.
In FIG. 29, let us consider a case where the magnetic recording
layer 10 has a linear shape (refer to the third, fourth and fifth
structural examples). The tunnel barrier layer 20 and the pinned
layer 30 are stacked on the magnetic recording layer 10. Moreover,
anti-ferromagnetic layers 71 and 72 are attached to the lower
portions of the first magnetization fixed region 11 and the second
magnetization fixed region 12 of the magnetic recording layer 10,
respectively. The anti-ferromagnetic layers 71 and 72 are connected
to source/drain diffusion layers of the first transistor TR1 and
the second transistor TR2 through vias 73 and 74, respectively.
Also, a shield magnetic layer 75 made of soft magnetic material is
provided at the middle of the via 74. The shield magnetic layer 75
has an effect of shielding a magnetic field with respect to a side
of the anti-ferromagnetic layer 72.
With regard to the structure shown in FIG. 29, a method of fixing
the magnetization directions of the magnetization fixed regions 11
and 12 to the opposite directions is as follows. Referring to FIG.
30, a first annealing process is first performed under a magnetic
field that is strong enough to exceed the shield effect by the
shield magnetic layer 75. As a result, both of the
anti-ferromagnetic layers 71 and 72 hold the magnetizations of the
same first direction. Next, a second annealing process is performed
under a magnetic field that does not exceed the shield effect by
the shield magnetic layer 75. The direction of the second magnetic
filed is set opposite to the direction of the first magnetic field.
As a result, the magnetization direction of the anti-ferromagnetic
layer 72 is maintained, while the magnetization direction of the
anti-ferromagnetic layer 72 is changed to the second direction
opposite to the first direction.
As described above, it is possible to fix the magnetization
directions of the magnetization fixed regions 11 and 12 to the
opposite directions by performing the annealing process twice with
the use of the shield magnetic layer 75. According to the present
exemplary embodiment, it is possible to fix the magnetizations of
the magnetization fixed regions 11 and 12 with such a simple
structure and by such an easy process. Accordingly, the area of the
magnetic memory cell and the manufacturing cost can be reduced.
7. Seventh Exemplary Embodiment
In a seventh exemplary embodiment of the present invention, another
example of the magnetic recording layer 10 having a U-shape is
provided. According to the present exemplary embodiment, the
U-shaped magnetic recording layer 10 is sterically formed. More
specifically, as shown in FIG. 31, the magnetization switching
region 13 is formed parallel to the XY plane, while the
magnetization fixed regions 11 and 12 are formed parallel to the YZ
plane. In other words, the magnetization fixed regions 11 and 12
are formed to be perpendicular to the XY plane. The magnetization
directions of the magnetization fixed regions 11 and 12 are both
fixed to either the +Z direction or the -Z direction.
The structure shown in FIG. 31 may be formed within a trench
section 16, for example. First, the trench section 16 is formed in
a certain layer by using the photolithography technique. Next, a
film as a material of the magnetic recording layer 10 is formed on
the entire surface. After that, the surface is polished by a CMP
(Chemical Mechanical Polishing). The magnetization fixation can be
achieved by any of the above-described methods. As a result, the
magnetization switching region 13 is formed on a bottom surface of
the trench section 16, while the magnetization fixed regions 11 and
12 are respectively formed on opposed side surfaces of the trench
section 16. Since the magnetic recording layer 10 is sterically
formed, the area of the magnetic memory cell can be reduced.
FIG. 32 shows an example of the cross-sectional structure of the
magnetic memory cell according to the present exemplary embodiment.
The magnetic recording layer 10 is formed within a trench section
76. The tunnel barrier layer 20 and the pinned layer 30 are stacked
on the magnetic recording layer 10. Moreover, anti-ferromagnetic
layers 77 are formed on the top sections of the magnetization fixed
regions 11 and 12 of the magnetic recording layer 10, respectively.
Wirings 78 are formed on the anti-ferromagnetic layers 77. Also,
the wirings 78 are connected to the source/drain diffusion layers
of the first transistor TR1 and the second transistor TR2 through
the anti-ferromagnetic layer 77 and via 79.
8. Eighth Exemplary Embodiment
The magnetization switching (domain wall motion) in the
magnetization switching region 13 can be assisted by a magnetic
field applied from the outside. For example, FIG. 33 shows an
example of a structure in which an assist wiring 81 for assisting
the domain wall motion is provided. FIG. 34 is a plan view of the
structure shown in FIG. 33. In FIGS. 33 and 34, the assist wiring
81 is so provided below the magnetization switching region 13 as to
intersect with near the central portion of the magnetization
switching region 13. Furthermore, the assist wiring 81 is connected
to the first magnetization fixed region 11. In the write operation,
the write currents IW1 and IW2 are supplied to or drawn from the
magnetic recording layer 10 through the assist wiring 81.
The structure of the magnetic recording layer 10 in FIGS. 33 and 34
is the same as the first structural example shown in FIG. 4.
Referring to FIG. 4 and FIG. 34, the first write current IW1 in the
first write operation is introduced from the assist wiring 81 to
the first magnetization fixed region 11 and flows toward the second
magnetization fixed region 12. At this time, a direction of an
assist magnetic field H applied to the magnetization switching
region 13 due to the first write current IW1 flowing through the
assist wiring 81 is the X direction. That is to say, the assist
magnetic field H assists the magnetization switching. On the other
hand, the second write current IW2 in the second write operation
flows from the second magnetization fixed region 12 to the first
magnetization fixed region and flows into the assist current 81. At
this time, a direction of an assist magnetic field H applied to the
magnetization switching region 13 due to the second write current
IW2 flowing through assist wiring 81 is the -X direction. That is
to say, the assist magnetic field H assists the magnetization
switching. It is also possible that the assist wiring 81 intersects
with the magnetization switching region 13 above the magnetization
switching region 13 and is connected to the second magnetization
fixed region 12. The same effect can be obtained even in that
case.
Also, in the case of the second structural example shown in FIG. 5,
the assist wiring 81 may intersect with the magnetization switching
region 13 below the magnetization switching region 13 and is
connected to the second magnetization fixed region 12.
Alternatively, the assist wiring 81 may intersect with the
magnetization switching region 13 above the magnetization switching
region 13 and is connected to the first magnetization fixed region
11. Moreover, the same applies to the cases of the third structural
example, the forth structural example and the fifth structural
example. The assist wiring 81 is provided above or below the
magnetization switching region 13 and connected to the first
magnetization fixed region 11 or the second magnetization fixed
region 12, depending on the structure. The point is that the assist
wiring 81 intersects with the magnetization switching region 13 and
is designed such that the assist magnetic field H assists the
magnetization switching. Consequently, it is possible to reduce the
first write current IW1 and the second write current IW2.
Also, as shown in FIG. 35, assist wirings 81 and 82 may be provided
below and above the magnetization switching region 13,
respectively. In this case, the assist wiring 81 is connected to
one of the first magnetization fixed region 11 and the second
magnetization fixed region 12, while the assist wiring 82 is
connected to the other. Consequently, the assist effect is
increased, which makes it possible to further reduce the write
currents IW1 and IW2.
Furthermore, the assist wiring 81 (or 82) may have a yoke wiring
structure as shown in FIG. 36. That is to say, surfaces of the
assist wiring 81 (or 82) which do not face the magnetization
switching region 13 may be partially covered by a magnetic layer
83. The magnetic layer 83 is made of Fe, Co, Ni or an alloy
thereof. Although side surfaces and a bottom surface of the assist
wiring 81 are covered by the magnetic layer 83 in FIG. 36, it is
also possible that only the bottom surface is covered. The assist
magnetic field is increased by such a yoke wiring structure, which
makes it possible to further reduce the write currents IW1 and
IW2.
Furthermore, according to the present exemplary embodiment, a write
margin is enlarged because the write currents IW1 and IW2 are
reduced. The reason is as follows. The write currents IW1 and IW2
need to be set within a range from a current Imin to a current
Imax. The current Imin is a lower limit current which allows the
domain wall to move within the magnetization switching region 13.
The current Imax is a minimum current which causes the domain wall
to move into the magnetization fixed regions 11 or 12. The
reduction of the write currents IW1 and IW2 means reduction of the
current Imin. Thus, the write margin is enlarged. It should be
noted that in the case where the magnetic recording layer 10 is
formed to have the U-shape as shown in FIGS. 33 to 36, the assist
magnetic field does not affect a domain wall motion within the
magnetization fixed regions 11 and 12 that should not occur under
normal circumstances. Therefore, the U-shaped magnetic recording
layer 10 is preferable from a viewpoint of the assist magnetic
field.
Also, the assist wirings 81 and 82 may not be connected to the
magnetic recording layer 10. In this case, currents flowing through
the assist wirings 81 and 82 are controlled independently of the
write currents IW1 and IW2. However, the configurations shown in
the foregoing FIGS. 33 to 36 are desirable from a viewpoint of the
number of wirings and a control circuit. That is to say, it is
preferable that the wiring for supplying the write currents IW1 and
IW2 to the magnetic recording layer 10 is utilized as the assist
wiring 81 or 82. In this case, increase in the number of wirings is
prevented and there is no need to provide a special control
circuit.
9. Ninth Exemplary Embodiment
It is also possible to write a data to the magnetic memory cell 1
explained in the foregoing exemplary embodiments by applying a
write magnetic field from the outside. In this case, the MRAM is
provided with a write wiring 90 that is magnetically coupled with
the magnetic recording layer 10 (magnetization switching region
13), as shown in FIG. 37. At the time of writing data "1", a first
write current IW1 is flowed in the +Y direction through the write
wiring 90. A first write magnetic field generated by the first
write current IW1 is applied to the magnetization switching region
13. As a result, the magnetization of the magnetization switching
region 13 is reversed, and the domain wall DW moves from the second
boundary B2 to the first boundary B1. On the other hand, at the
time of writing data "0", a second write current IW2 is flowed in
the -Y direction through the write wiring 90. A second write
magnetic field generated by the second write current IW2 is applied
to the magnetization switching region 13. The direction of the
second write magnetic field is opposite to the direction of the
first write magnetic field. As a result, the magnetization of the
magnetization switching region 13 is reversed, and the domain wall
DW moves from the first boundary B1 to the second boundary B2.
* * * * *